Have your friend move out! As soon as possible. But that is only the first step, and you have to make sure it works - so prepare before acting:
It sounds like that situation is in fact life-threatening, and that calls for serious action. The first step would be to get out harm's way immediately, and that in itself isn't as easy as just getting up and walking out the door because the parents are likely to "hunt" their daughter outside the home, so a lot of additional steps and external help needs to be tied into this first step.
I am not skilled in handling such situations, but there are shelters and hotlines that can assist. Perhaps it's better to prepare in silence, to research, to talk to services -- and only openly act once you're reasonably sure that the action will save the girl. I could imagine that a "failed attempt" might make the situation a lot worse.
As her friend, you could contact such services and get some anonymous advice. At the very least, it would help you to cope with the burden of "knowing but not being able to act".