It's not sitting back and doing nothing. It's being there for that person and not judging them every time they go back to him or take him back. It's hard not to do that, but once you do they will shut down and you will never be able to help them. It's letting them know that when they are ready to leave, you are ready to assist them and give them a safe place to go. They need a lot of counseling.
Of course, it's different if it's your sister. My brothers would put the guy in a hole and he wouldn't be heard from again. I get that. But the problem is not every guy in the world is a tough guy. Not every guy is supposed to be. Not every girl in the world has a brother. Women shouldn't need men to be safe from men. But we all need a support system. It's a really tough situation. There's no easy out. I think people always try to make it a lot simpler than it is. It does sound easy, oh just leave. I once had a fight break out on my fb page about this. One of my friends said she didn't feel sorry for Rihanna because she went back to him so now what ever happens is on her. And another friend of mind got really mad and told her off in the context of her sister coming very close to being killed when she tried to leave her abusive husband. It's not like women get this any better than men. In a lot of cases, they don't. The first thing I think anyone should do if I they have someone close to them in this situation is reach out for professional guidance, counseling, and help.