I was quite norty with my friend who like yours was in a terrible relationship with a controlling man who used to phone her every 5 minutes to see where she was and asked to be put on the phone to me if she told him she was at my house just to make sure. (We all knew he was cheating on her and was checking that she was no where near where he might be playing away)
Anyhoo I was very pensive one lunch date and she asked what was wrong and I told her that I had a friend that was in a terrible relationship and listed what I thought was wrong. I told her how much it upset me and how uncomfortable I felt because she couldn't or wouldn't see him for what he was. I asked her for her advice as I said I was stuck as to what to do mainly because he'd managed to exclude her from all of her friends and was making good inroads into her family and had almost totally eroded her self respect and self worth. Sooner or later it would mean she'd be left with no one and he'd have her solely to himself. I just kept asking her what would she advise?
Her advice was that my friend would shoot the messenger and not want to hear what was wrong with her hubby as this person would have made my 'friend' believe he was only looking after her interests and any other friend would be jealous of their closeness. So I said I couldn't stand seeing what would happen so I would just stop the friendship.
It took her another 3 months to realise I was definitely talking about her as I kept making excuses not to meet up with her or visit her and thankfully she saw sense and eventually got rid of him although in truth the longest day his bumhole looks down, she'll never be rid of him but that's a different story
I'd say do nothing but be there to pick up the pieces cos this guy sounds like an emotional abuser who wants to cut her off from everyone and that's when I hate to say it, it can change from emotional to physical abuse