Go for it!! Just make sure the letter is strictly professional and not personal. I was in the same situation. I had a reference that was a friend but also someone I worked with professionally, so I asked her to write a professional LOR. I was accepted to the program she sent her letter to.
I've heard back from a couple of schools and have been accepted to BU and UW, and rejected from UT Austin. I'm still waiting on CSULB and CSULA. UCLA is my top choice though. I've heard nothing yet ? Maybe this evening. Fingers crossed!
I'm pretty sure that they save rejections for last. So if you haven't already been accepted, they're most likely not sure about you or are gonna reject you. Someone please tell me I'm wrong ?