MSD

For her sake, you need to get her thrown in jail, which will be followed by court-mandated rehab. It's the only chance she has other than killing herself with self-destructive behavior. Your other choice is to wash your hands of the matter, leave her and her friends alone, and let whatever happens, happen.

You have no moral obligation to do anything, but if there's any compassion left in you, even for someone who so severely abused you, you should say something that will lead to her being arrested. It can save her life.