The primary goal of any parent is to protect their child.
In my childhood era the parents of "best friends" became fairly acquainted with one another. There was a bond of trust built between them which allowed them to be put at ease when the children had sleep overs.
There is no way under that scenario if one parent knew the other parent's child was being abused that they would keep their mouth shut about it. Many of today's parents are more interested in being their child's "best friend" then they are being the responsible adult.
A 15 year old does NOT know what is in their best interest! Clearly she wants to keep the abuse a secret because she feels she is "in love" with her abuser. The time is now to break up this cycle of abuse. If she accepts this crap at age 15 what will things be like for her at age 25? Keeping quiet about abuse you know is taking place is the same as being a "silent accomplice".
Suppose this boyfriend kills the girl? I would hate to be the parent who knew of the abuse. I can see the police questioning the best friend and her stating "I told my mom/dad about it." How would one justify their silence? A parent should always do what is in the best interest of the child even if it will piss them off.
I would contact this girl's parents and tell them I have some disturbing news that I thought they needed to be aware of. I would then prepare my child for the possibility that her best friend may pull away from her. That's life! Sometimes doing the right thing may yield some negative consequences but in the end one's soul remains intact. Last but not least I would tell her:
"Never love anyone who treats you like you're ordinary"
- Oscar Wilde
Clearly if someone is abusing you they don't think you're "special"! Being loved shouldn't hurt!