Hmmm. Things like this are always tough to deal with regardless of the woman's age. I have a few friends who are in similar situations, as well I had experience with an abusive and controlling relationship so I acknowledge the severity of this issue. All I could suggest is a mentor, a friend or someone who this girl respects would offer to do some non-professional counseling sessions with her. I would suggest therapy or group therapy specifically pertaining to the issues of battered women, but that may not be something she is willing to participate in, and I can bet the asshole will discourage her from doing so.
The only cure to this situation is her realization and a clean cut break up with the guy, and that is most likely something she is afraid of. What makes this situation so hard to "control" from an outside perspective is the more you try to stand in the way of her emotional attachment to this guy the more she will put herself in danger by pulling away, and running to him, this can lead to him turning her against her parents completely, and then the likelihood that she will be further victimized is greater. The best thing to do is to offer the girl a haven, someone to vent to when they get into fights, and take that time to plant the seeds of doubt, and more importantly options to leave.
Most girls who get stuck in abusive relationships eventually break up with them, but all too often only after they are put in the hospital and all their friends and family are alienated. This is what happened to me, I was stupid enough to bail my abuser out of jail 3 different times after he was arrested for DV against me. The last time I landed in the emergency room with 2 broken ribs and a serious concussion. Maybe it was the hit to the head but it just clicked, I HAD to escape this man's hold on me. Something serious like this might have to happen for her to fall out of love with him... unfortunately sometimes young love can be too unconditional and it takes something really bad to break that obsession.
Forcing a child into treatment for this issue just wont work, shell get out and call him, believe me I was int he program for 2 years and beyond my better judgement I called my ex (not the same guy I mentioned, but an asshole all the same) You cant lock her up to prevent her from making mistakes, she has to make them and learn from them. Life is the ultimate "tough love" and really the only one that is effective.
All I can say is she needs a friend to confide in, one who can encourage her to leave him. Maybe even a male who she might become romantically interested in that way it gives her some incentive to leave the asshole. I don't know, just throwing out options lol.
If you think the girl will talk to me I'd be willing to give her some advice, pm me her email or myspace and I'll try to contact her.