From what you share, you are battling with your feelings that you care and want to help her, yet at the same time it's starting to take a toll on you. It's ok to step back and take care you yourself. We cannot really stop someone else for making bad choices. If this man is abusive she has to report it and take steps to get help.

Allowing him to come back into her life when he can be abusive is a bad choice on her part.