I grew up in a house of emotionally and sometimes physically abusive women family members (mother included). Even though that's not the exact same situation here, I did learn a lot about how to deal with abusers.

The first thing is that hatred of the abuser doesn't solve the problem, and only makes us miserable. I agree completely with not spending time with your friend when he is with his abusive g/f. You have to tread carefully here, as it is of utmost importance that you are available and supportive of your friend. If your friend feels judged/rejected or disapproved of, he might respond by withdrawing, and that you DON'T want. You need to keep him close. You also have to get him alone and have a discussion with him about a course of action. This is NOT healthy for either him or the girl, who She obviously has some serious mental health issues going on that need to be identified and treated anyway. She might really be self-destructive, even if she does use this as a manipulative tactic to keep her b/f. Since we don't know for sure, think of safety first.

I would suggest telling your friend that he tell his g/f that she needs to go to counseling, and that if she doesn't agree, then he will leave. The obvious response she will have is to threaten suicide to manipulate him into staying, to which his response will be to call the police and report that his g/f is a danger to herself and possibly others and threatened suicide. If need be, he could file a restraining/no-contact order while she's still being held in psych (most states 48-72 hour mandatory stay). This really is the only right thing to do here. One must always assume the worse. If he knows her parents or siblings, your friend could also call them up and tell them that he is worried about her behavior and that she is threatening suicide. They might be good backup or they might not, depending on her family situation.

That will not only give him time to develop a strategy on how to leave her without getting hurt or falling into the same old pattern when she gets out, but it will also help to eliminate guilt and fear on his part---he will have done all he could have done by reporting her suicidal threats, and effectively be of clear conscious. The main thing to watch out for is to make sure he doesn't get suckered into seeing/talking to her while she is in the psych ward, as she will no doubt lay a guilt trip and manipulate him into her bidding again. The other thing is to make sure that he doesn't see/speak with her once she gets out.

Have him change his phone numbers (cell and home) before she gets out as well. Anything he can do to create distance is a good thing. You'll have to be really supportive toward him during this process, as his resolve and strength on this matter will probably not hold without it your support. With his self esteem being so low, I'd really recommend that you try to get him into a psychologist so that he can undo some of the damage that his abusive g/f has done.