Craig, I hope you aren't feeling overwhelmed by all the advise being offered here. But, it is good advise and if you are her best friend, you can't just let her figure it out on her own.
Have you talked to your own parents about this? This is a big burden to place on a 15 year-old's shoulders, so you should talk to them, if you haven't already. They can make phone calls for you, if you aren't comfortable doing it yourself.
Living with a boyfriend may get her out of an abusive household, but that is - at best - just a temporary measure, as far as I'm concerned.