Help! My sisters husband is a nut job and I wish she could figure it out.?

I don't know what to do. My sis and her husband separated a couple of months ago. He was sleeping with other women. They have two kids... been together forever... She's talking to him again. I think she's an idiot. Two days ago she was very upset because he told her that she needed to get rid of eleven of her facebook friends if she wanted to stay with him. She said his reasoning was that they were connected to the guy she started seeing while they were separated. She doesn't have that guy on her facebook though... and she made some very good friends with some of his friends. That's why she was so upset. I told her that this is very controlling and that people have friends tied to bad dates all the time... and friendships last forever. She freaked out when he said he was done though and deleted everybody. I was hoping that maybe I could show her posts on here and show her that it's not just me... Can anyone help me? Is it right for her to break all ties with friends just because they met through a guy that she saw because her husband told her too? Or is it okay for her to be friends with friends when there really is no tie to that guy except that she met her friends through him? He has beat her in the past. She said it was because he found out she was seeing someone else while they were separated and that he has changed and that he swears she is it and that he doesn't need to see other women anymore. Except for this separation when she found out he was seeing the babysitter they have been together for seven years. She bulked when this guy she was seeing showed his true colors and she called her husband right after that. I think she's just afraid and I know she wants to be the good wife and not be a statistic... I have never had a major relationship... I think that that's part of why she won't listen to me. Can someone please put some advice on here that I can show her... I'm really worried about her. Am I being an unsupportive bad sister or does she need to drop this jerk?