Hey,
I was in the same dilemma but at 16 I was dating a 23 year old who was really sweet and became abusive mentally and kept blackmailing me cause I cut off all my ties when I was with him. My bestfriend, you in this case gave me an ultimatum either I tell my parents or she does, you have to help your friend, I got into a even stickier position he basically raped me and I got prego but I kept putting up with the bs that is why my friend got involved.
My parents picked me up(more of dragged me out)they were pissed at first but they helped me more than you can imagine, I told them everything I had done and they forgave me but it took a few days to calm down. Me and friend still talk and I appreciate what she did it was for the better i'm 21 now and i'm married and expecting a #2 but my husband is my bestfriend now and he knows everything about my past so that none could blow up in my face..
I wish you the best just help her before it becomes too late, what he is doing is a form of abuse and her parents will love her no matter what is said. If you need to ask anything send me a message
jess