Of course this girl shouldn't have turned others against your daughter and should not be violent towards her but some of your post does concern me as a parent whose child once self harmed.
Why would your daughter fear that you would stop a friendship due to self harm?
You do know that self harm is the most common form of abuse don't you and that lots and lots of teenage children do it.
You do know that you should have absolutely no idea what the school is doing about it and what the counselling situation is don't you.
You do know that this girl is vulnerable and that you should never ever have disclosed something told to your daughter in confidence to her mother?
You do know that self harm isn't a disease and it doesn't make a person bad or even mentally ill.
Contact the school to support your own daughter and ask for some assistance and information about self harm, it's incidence, and why teenage girls do it.
Please don't think badly of girls who self harm. It isn't sinister. My daughter did this in Yr 7 because she was so stressed by the transition to secondary school and the behaviour of some of the other girls there and scared she would be bullied by them. She felt totally out of her depth. She forgot her PE top one day and one of her friends and a teacher went to the Head of Year. It all spilled out later that day and we just hugged and I made a doctor's appointment where she confirmed why she was so upset - in private without me in the room. After that some adolescent counselling was arranged and all has been well since. My dd is 17 now. She doesn't have mental health problems, she has had a very successful school career and is looking forward to applying for university. Fortunately, other parents who were told were very very supportive and concerned for her and us. Nobody contemplated telling their daughters not to be friends with mine because she had a vulnerable patch and self harmed - quite the reverse in fact.