You can't tell her to break up with him. She needs to make that choice herself. If she is suffering from bad parents, she may have a problem with self esteem. You can do things that will boost her self esteem, plant subtle suggestions that there are better people than her boyfriend and maybe even present some of these better people to her, without her boyfriend around. To force her may backfire and she could end up with a similar string of loser after loser. When I was young, I never listened in these matters, but I can tell you I wish that I had.
Keep contact, be very warm and supportive of her, but don't bring him up unless she does.
Abusive partners tend to deliberately drive away friends and family in order to better control their partner/victim, so don't play his game.
You can't. She's an adult and she has to learn in her own time. She will leave him sooner or later, so don't interfere unless she comes to you asking for advice.