That sucks. A lot of girls take loyalty to their boyfriends to an extreme level - I had a boyfriend stop a man from beating his girlfriend in a parking lot, and the crazy bitch turned on my boyfriend. Also, it is well-known that domestic violence cases are one of the most dangerous calls a peace officer can take.
Your friend considers her relationship with this guy to be more important than her relationship with you, although she may feel bad about this. Because of this, I wouldn't get involved in anymore of her relationship problems. She is obviously willing to tolerate crap out of him, so putting yourself in the spotlight like that is non-productive.
Sorry, my friend.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?