If I were you I'd move on and let her deal with her decision. And to answer your question, I have gone back to an abusive ex and on a drunken night he strangled me unconscious. I was in a neck brace for a week and haven't seen him in 4 years. I hope I never see him again and I'll be damned if I give another abuser the chance to hurt me. Even if they're drunk... That doesn't mean they won't eventually act out down the road sober. People like that are a ticking time bomb and only time will tell. But this relationship is really not your business and you should just let her go to figure it out herself.