Hi M.-
Several years ago, when my oldest daughter was about 14, I was (truly) dusting in her room, and came across a note to a friend of hers regarding 'cutting'. It was evident from the note that my daughter knew her friend was doing this...and was trying to 'help' her friend.
My daughter was initially VERY angry with me...and accused me of 'snooping'...(I was not...but having seen what I saw...HAD to address it). I discussed it with my daughter...and arranged a time for both my daughter and myself to talk to her friends mom. It was awkward to say the least. The mom was in denial at first...and many tears later for ALL of us....there seemed to be some 'movement'.
For a long while my daughter and her friend's relationship was strained...BUT...the mom got her kiddo into therapy...and things improved all around over time.
My daughter is now 20...and she (and friend) are both juniors in different colleges...they are still in touch...and see each other sometimes over breaks...
I used this as a 'teachable' moment...NO ONE should feel obligated to keep a secret when someone is in a great harms way.
I would tell...and I would expect the 'worst' in terms of hurt feelings...and 'noses out of joint'...
BUT...in the longer term...you might prevent something very much more serious and devastating from happening...
Just MY opinion!
best luck!
michele/cat