I frankly think if you think she is trying to tell you something and you want to be there and listen, then do just that. If he is being controlling of her then it's not going to be so easy for her to admit the situation is wrong or that she wants out. She may have been in abusive relationships in the past, she might be used to allowing men to have what they want.
A lot of women cave in from guilt, fear, shame, or because they think they deserve it. There is also the matter of manipulation, he might be putting it into her head that she deserves it, or that she can't get better, or that he isn't doing wrong.
May family has a history of abuse, and I am the only female who hasn't been in an abuse relationship (although I have been abused) because of my fear of ending up like that. This situation is not uncommon.
While you can't save her, and I know full well the frustration you are going through, if you want to support her then do so.