I would move out. With people who act this irrationally you never know what is coming around the next corner. Just think about it, she is pregnant her hormones are probably going to start going out of control and making her act even more irrationally. They will continue to fit, he will kick her out, then let her back in, kick her out again, etc.
I would take your other friend up on the offer and move into the house. You just have to be firm but honest with your current roommate, explain your moving out because with everything going on with him and his girlfriend you feel it would be less stressful for everyone if you moved. They can get privacy to work on their relationship and you get quiet space to relax. Personal, I would have said something when you found out this girl is a pill-head. No way someone doing drugs is staying in a place I am paying rent for.