I would make sure your friend has a therapist and a psychiatrist.
Gently I think you should stress that she should not be doing this, and for her to substitute something else that makes her feel better.
I dont want this to sound trivial, but I am bipolar 2, it's pretty complicated but vigorous exercise, every day if necessary, helps me a lot. It's a distraction, it gets the blood flowing for clearer thinking, it releases feel good hormones. I am not talking about talking a walk, tho even that is helpful. I am talking about biking, water aerobics, something that really gets the blood pumping.
I had a best friend who was a cutter. She really needed activities to build her self esteem, things she could do well, but her family wouldnt let her do anything.Good luck. I know this is difficult.
Hugs!