my own advice would be to stop pushing the issue. a friend intervention will probably make her even more upset, given that she doesn't like it when you bring it up by yourself.

i remember when i was in high school, and i knew i had to stop, but the thought of stopping was so scary because i didnt know how else to cope. so maybe that is a way you could approach it, if you must - by brainstorming other things she can do instead?

but i think, given that this is tearing you apart atm, maybe you should tread gently and follow her lead when she brings it up. encourage her to seek help when she brings up the idea on her own, but just be her friend otherwise - do the things you would usually do together etc.

is she still living with her father? or anyone else who is a threat to her?