falala

I'm looking for some advice about how to deal with a really awful situation that's come up in my program. Sorry that this is basically a novel, but please, I would really appreciate some help. Context: I'm in my second year, and I have two really close friends, A and B. They have both had a really hard year in a number of different ways, both academically and personally. They live together. This year, C and D entered the program. They live together, and while I originally thought I would be friends with both of them, C has turned out to not be a great person at all (D, however, I really like). C is basically an emotional sponge; she is always asking people to do things for her and asking for advice without ever giving anything in return. She's the kind of person who ask you how you are so that you can respond, "Fine," and then she immediately launches into a long rambling discussion of her emotional state. She loves to talk about herself, and seems to think she can never do anything wrong. Whenever something goes wrong in C's life, it's always someone else's fault. C has tried to be friends with everyone in the department, but most people are pretty fed up with her (I've heard about 90% of the department discuss how annoying/irritating/infuriating she is when she's not around). She's also had conflicts with professors and hasn't always fulfilled the requirements of her teaching (although she loves to tell everyone how amazing her teaching is and how she was a born teacher and....). She's smart enough to finish the program, but given her conflicts with others (especially professors) I'm not sure she actually will. The actual situation: C had sex with A while A was blackout drunk. The short version: C told D the day after and tried to get D to absolve C of the guilt she felt, including C changing her story about how drunk she was and how she navigated consent with A. A few days later, D told A what C had said, and A said she was blackout drunk. A told us she was scared of C and wanted C to stop telling people in the department and to have C stop talking to her. B and I confronted C about what happened, asking her what happened that night. C admitted to use they had sex. When we told C that A had been blackout drunk, C got mad at us for insinuating that C had sexually assaulted A. We asked C to leave A alone, which C said she would do. This happened Friday, and it's unclear if C will actually honor it. The long version: C and A went out out for drinks on a weeknight a few weeks ago near the university. I wasn't there, but A said that C was trying to flirt with some random guy and get him to come home with C. A could tell the guy wasn't that into C, but tried to be a good wingwoman. C ultimately got the guy the agree to come home with her, but C was worried that her roommate D would judge her for bringing a guy home, so C talked A into letting all three of them go back to A's place. B heard A and others come home around 3am that night. A remembers discussing going to her house, but nothing after that (A is working on her drinking, but does regularly get blackout drunk. I saw her the next morning on campus at 9am and she still smelled drunk, so I fully believe that she was black out drunk). B didn't realize there were multiple people at her house until the guy tried to come into her room instead of A's room. B told the guy he needed to leave. When B went into A's room to tell her, B found C completely naked in A's bed and A seemingly passed out with most of her clothes on. B said that C seemed pretty alert; she pulled up a sheet over her body pretty quickly and seemed cogent when B told C that the guy needed to leave. After the guy left, B went back to bed and didn't hear when C left, although it was sometime before 7 or so. B also texted me to tell me about the guy and seeing C naked. D told me, A, and B what had happened the next day with her and C. All of this paragraph is according to D, but I believe everything she told us. D ran into C the next evening, and asked C where she had been out so late. C immediately told D that A and C had sex the night before. C first said A and C had both been drunk, and D was a little uncomfortable with that. When C saw that discomfort, she changed her story, saying that they were both fine to consent. When D asked C how she knew that A was ok, C said that she had checked-in with A as things happened. C had another friend there as well, who told C that it sounded like C had done everything right. D did not say anything like that, and generally thought that C was trying to feel better about what happened. C also told D about B seeing C naked, and specified that A had been fingering C when B walked in (B said C and A weren't even touching when she came in). D felt uneasy about the conversation, and later told her partner that it sounded "rape-y". D told us all of this a few days later. My immediate thought was that C was lying about having sex with A; B agreed with me. C and D have some weird tension where C is always trying to one-up D, and C knew that D found A attractive, so it seemed like C might've lied to try and show off to D. D said that she knew C had to be lying about some of it, since parts of it were inconsistent with one another and what B knew about what had happened. A was unsurprisingly really upset by hearing about this. A couldn't remember what had happened, but knew she was not interested in C and would not have consented to sex with C. A didn't express an opinion about whether or not C was lying, but I think she believed it had really happened. A told us that C had texted her the next day, asking if A was ok. They had texted back and forth a little bit, but C didn't admit anything sexual had happened, and it sounded to me like it could've easily been about whether or not A was hungover or not. After A found out what C had said had happened, A was super uncomfortable around C, to the point of having anxiety about seeing C. A tried to avoid C and the office for a few days. When A told B and me about how she was feeling, we tried to get A to talk to C with us. Again, I really thought C was lying, and it would be good for A to find that out. We were pretty sure we could get C to leave A alone by confronting her. A was reluctant at first, but agreed that we should talk to her. She kept changing her mind, though, and ultimately we decided that B and I would talk to C without A, but with the clear message of getting C to leave A alone. B and I spent a week trying to find a time when us and C were in the office. But we had somewhat opposite schedules, so whenever B and I had time and would try and talk to C, C would say she had somewhere to be. Ultimately, I emailed C asking for a time to meet with me and B, since trying to find a time hadn't worked at all. C agreed to meet with us the next day via email. However, C then saw me a few hours after agreeing, and demanded I tell her what the meeting was about. I was working on something when she started talking to me, and I kept working, while saying that we would talk the next day. C kept bothering me, so I told her I wasn't comfortable talking without B. C got mad and told me that she didn't want to talk if it was "going to be like this" and wandered off. C emailed us the next morning to say she wouldn't meet. I saw D that day, and she told me C had said that I had been rude, so therefore C wouldn't talk to me. I apologized to C via email in an attempt to get her to agree to meet. She sent me an email back to just me (I had been cc'ing B on all the emails) saying that she was sorry if she had hurt my feelings (for something she had said a few months ago that I had told her was inappropriate) and that we hadn't talked much (I avoided C like the plague, generally) and that it was weird that I had emailed her about meeting. I got her to agree to talk with us at the end of last week. I had made all these plans to be totally calm and ask a bunch of questions in the meeting with C. Of course, it did not go that well at all. B and I started by asking what had happened that night with A. C immediately told us she wouldn't tell us, because it was "personal". I had been hoping to get some idea of what happened before telling her A had been blacked out, but to get C to even say anything, we had to tell her that A didn't remember the night at all. C said she was surprised to hear that, but wanted to know why she had to talk about it with me and B, not A. I was hoping not to have to immediately tell her that A was scared of her, so we told C that A was uncomfortable talking to C. C tried to walk out at that point, saying that if A was uncomfortable, C would just start the conversation, so we told C that it wasn't just that A was uncomfortable, but that A was scared. C said she didn't understand why, so I asked her, "well, A has known she was blackout drunk and that D said you were saying you'd had sex. What do you think A has been thinking?" C got pissed at that and called me condescending. C told us that everything she had said to D was true, and that she'd told a few more people in the department about what had happened as well. We told her that A did not want to talk to her, or for C to keep talking to people about it, including people outside of the department. C said she could do that, and asked if there was anything else. I reiterated that trying to talk to A or talking about the situation would hurt A, and she again said she wouldn't. I think that's probably a lie; I'm pretty sure C went to immediately call a friend, although I hope it was at least someone from far outside the department. At no point, though, did C express any regret or sadness or anything about what had happened; she was just mad at me and B. A is still scared of C, but doesn't want to get the university involved at all. A doesn't trust that they can actually doing anything to C, and A is also worried about backlash from people in the department (we're pretty small, about 20 grad students). I'm planning to call the university safety hotline, because I'm scared for A at this point. But C is someone who is going to be in the department for the foreseeable future; we're on the quarter system, so the school year isn't even over. Her workspace is pretty close to mine (and A's and B's; we asked her to consider moving hers, and she said she'd 'consider it once the year was over'). D is also afraid of her, and D has to live with her; I encouraged D to call the safety hotline as well and she is willing to do so, but hasn't done it yet. I have already been largely ignoring C, but I'm worried about what she's going to do. I want to get the DGS or chair involved and I've tried to convince A that it's the best thing to do. But A will not budge, and I can't explain what's going on without giving away some details that A does not want shared. I want to respect A's wishes, but I also know there are some things that it's not okay to keep secret. Anybody have ideas about how to deal with this? I really have no idea how to deal with having an unrepentant rapist in my department.