What a heartbreaking situation to see your daughter suffer. Fortunately, however, I believe I can give advice on this situation. Being a person who has in the past, suffered from unhealthy relationships, I would suggest a tactic I find effective.
The first is to have your daughter take just 2 weeks off from this “relationship.” In that time, have her utilize dating sites to meet men who wish to treat her well. Doing this will cause her to truly realize that she deserves better than what her current situation is giving her. After the 2 weeks of absolutely no contact, she can reevaluate her situation. Chances are she will feel much stronger and choose not to have this terrible person in her life anymore.
Sites I would recommend would be okcupid and Craigslist. Of course, because these are strangers, she would meet in a public place and be wary of her safety. I would be more than happy to know how this works out for her. If you would be so kind to inform me how things work, I would be thrilled. I'm atXXX@XXXXXX.XXX. This means a lot to me, because as I had mentioned, I endured a very unhealthy relationship, and I don't want to see any person suffer in the same way.