I'm going to tell my best friend's parents about something she didn't want to talk about
Why are you telling her parents?
It is important they know.
Normally, it isn't nice to tell secrets.  However, you should if the person is in danger.
I believe she is
I really appreciate you love your friend that much you are willing to lose her to keep her safe. Even if she gets angry with you just know you did the right thing.