I don't know what I would do.
In my job position I sometimes hear things I really wish I didn't know.
Yesterday it was a male and female newbies - bf/gf but she was married.
I spent the entire day clenching my jaw so hard it hurts today and I have a tooth ache.
I spent my evening at home thinking about me being the bs and wh's HR person knowing and me going on about my life clueless for a year.
I was going to post about it but then I didn't really know what I expected SI'ers to say.
If I had a "good friend" doing this shit I'd probably be flipping a nutty on her and saying WTF!!!!!!!!!!!! I pull no punches with my good friends. Might be more productive if you try to help her through how unhealthy this is or get her to seek help than tell the BW.
And I can't believe I said that! But if it was me I'd want my friends to smack me smart before causing that kind of devastation.
But if I was the BW I'd want someone to tell me but then I read so much about how the bw's or other bs's don't want to hear, or know and get mad and don't want to hear it from other people.
So I don't think there is a right answer here.
But for sure if your friend is a good friend I'd tell her how you feel. Might give her a nice visual of what SHE might go through some day if karma exists.