Something important to realize I think is cutting is frequently done as a maladaptive coping strategy. I think something crucial your friend should acknowledge is cutting can be a very difficult habit to abstain from, especially if her daughter is going through a stressful time and doesn't feel comfortable talking with her mom and therapist. I haven't cut in nearly a year, but when I'm very stressed out, the urge to cut comes back in full force.
I think an important part of therapy is learning how to better cope and become more resilient with one's problems. Ideally, I think her therapist should be working towards identifying K's reasons for cutting, what triggers her to cut and what alternative coping strategies she can use instead of cutting. In order for this to happen though, K has to at least want to stop cutting. Wanting to stop cutting doesn't mean she still won't cut, but it at least means she is willing to work with her therapist to change her behavior.
There are a lot of reasons why people may cut:
- To externalize and/or to distract themselves from the emotional pain they're feeling.
- To punish themselves when they feel guilty or worthless.
- To feel alive when they feel empty or emotionally numb.
- The sight of blood and/or rush of endorphins calms them.
- To gain a sense of control when one otherwise feels powerless over their life or emotions
This list isn't inclusive but hopefully it helps you with understanding why people cut.
A particular book about cutting I recommend reading is Skin Game: A Memoir by Caroline Kettlewell.