First of all, you need to sit down with her and have an extremely calm conversation with her. If you get all emotional and start crying, or worse yet, yelling at her and telling her how horrible her boyfriend is, she will get defensive and probably try to stay with him just to spite you.
So arm yourself with some good information and be prepared to offer her a place to live and to help her move out of their shared apartment (I’m assuming they live together, right?). Because she will use those things as “reasons” for why she can’t leave. She might say she has nowhere to go (give her a place to go). She might say she can’t move her stuff easily by herself (get a group of people together in advance that would agree to help her move).
Then, invite her over to your house, or a nuetral place like a diner or coffee shop, where she’s less likely to start having a screaming fit in public. Then give her a print out of something like this How to Spot Signs of An Abuser and give her some clear examples of how her boyfriend may have hurt her or manipulated her or lied to her.
And do what @JLeslie suggested. Ask her what she would want you to do, or what she would want her best friend to do, if they were in an identical situation. Would she be OK with letting you or her best friend stay with an abusive man?
Let her know that even though you realize that she has deep feelings of love for this man, that love is not enough to sustain a healthy relationship, and that she can’t fix him. Women have been trying to fix men for thousands of years and it can’t be done.
Point out to her in a very calm tone of voice that her boyfriend is not so different from thousands of other abusive men who have wives and girlfriends who try to fix them or change them. And just like all of those other women, they end up getting hurt emotionally or physically down the line too. Tell her, “Don’t allow yourself to become one of those women.”
Tell her that you’re considering going to a support group for parents of females that are in abusive relationships, because you not only want to help her you have to help yourself to be able to cope with this terrible situation.
Ask her if she’d be willing to go into couples counseling with this guy (I’m guessing that she’ll say no because she knows that the guy would never agree to it). Ask her why not? Whatever kooky answer she gives you, you must reply to her, “Well honey, if you and Rex are not willing to go to counseling together, then there’s really no hope for this relationship. He is damaged, but if he’s unwilling to change and get the help that he desperately needs, you can’t fix him.”
And then tell her that she would be doing herself a huge favor, if she went into some short term therapy on her own, to learn new techniques on how to cope, because right now, she’s not coping, she’s simplly giving into this guy, in hopes that she can change him or fix him, but that is simply not possible.
Be prepared ahead of time, by talking to your own doctor and having your doctor give you some suggestions on what type of therapy is available for your daughter. Don’t just tell her she needs therapy, you need to give her concrete information on where to go and who to call and possibly give her some finanancial help to pay for the therapy. Otherwise she’ll just say no and that she can’t afford it.
If none of this works, you may have to arrange an intervention. My family and I had to do that for one of my female cousins. Her husband had verbally abused her for years and isolated her by moving to another state and not allowing her use of the car or telephone, and then we found out that he was physically abusing my little 5 year old nephew. That was the last straw. I will advise you, that if you are going to stage an intervention, do it with a professional, rather than the way we did it with just our relatives. It got ugly. A professional can help you to say the right things, to help your daughter in a safe and legal manner, and he can help you prepare a plan, in advance. If you and your relatives just drag her out, you have to have some sort of a plan in place for where she will live, how to get her things, whether or not any legal action will be taken against the boyfriend etc.
If, after you’ve staged an intervention and your daughter still won’t leave. Let her know that you are ready to accept her into your home and to help her leave, whenever she hits rock bottom. Then leave her alone and let her come to you, but leave the lines of communication open to her, so that she can contact you when the time comes.
Here is a site that discusses the Types of Therapy Available and How to Find a Therapist
Here is a site that discusses How to Leave and Abusive Partner at the bottom of the page is the phone number for the National Domestic Violence Hotline 1–800-799–7233
And here is a bunch of Links to Sites that can offer Help to Battered and Abused Women
So spend a day or 2 and get your plan together. You might want to talk to a few of your daughter’s friends, because you will probably need them to help you, and to offer additional support for your daughter. Good luck : )