Another cutter here. I began cutting when I was 12ish after I was sexually assaulted and my mother told me I was a whore for it.
For me, it was a way to control, it was something that *I* could do and absolutely NO ONE could tell me how the hell to do it. I would cut every inch of my body, even my face. I would carve words (help me, no fear, F/U, eat me, etc) in my arms/legs.
The biggest misconception about cutters is that we DO NOT attempt suicide. It is never a suicidal attempt when cutting, but sometimes it goes too deep.
I agree also with the therapy. There is an underlying issue with her. Something has happened in her life that has caused her to feel 'less than', 'out of control', etc.
I would seek a professional (if that is the route you take) that specializes in cutting.... THAT is very important. Cutters are misunderstood! Many ppl think they cut to hurt themselves in a negative manner, to try to die. This is not always the case. It, for me, was more so of being in control of my life, my choices, my decisions, MY BODY!!!!!
IF she has anyone in her life, mature/adult/role model type person, that could speak with her, I strongly encourage that. It *is* possible for her to get through this with out being seen by a therapist and ending up with a "label/metal illness record" for the rest of her life.
Wishing you the best in this journey! It may take many years of continuous help/encouragement/love/commitment to help her through this. She NEEDS support. I would also like to stress, PLEASE do not ever blame her for cutting herself, although it is HER who is cutting HERSELF. Cutting is a disease, just like alcoholism/drug addiction/porn-sex addiction... It is a disease. It DOES NOT mean she is nuts or needs to be put into the hospital for mental illness. She just needs help dealing with and healing from a trauma that has happened to her. It may take some time for whatever happened to her to come out, when it does, she will need all the supportive persons she can get to help her through it!!!!
You know her much better than ANYONE OF US DO. You know who she is, what she is like, what makes her tick, etc.
Subbing in order to read up on any updates... again, I wish you well in this journey!