She makes multiple scratches on her arm on a daily basis with a knife or scissors. Some psychological disturbance is causing your friend to do this. Self-mutilation occurs mostly in young people, and it is three times more common in women than in men. They could also be suffering from an eating disorder.
Some teens say that they feel numb, unable to feel or experience anything.
Your friend's self-mutilation is her way of asking for help. She needs to speak with someone not only to help heal her visible wounds, but her invisible wounds as well. Tell her of your concern, and see if she'd like you to accompany her to a counseling service. Your willingness to go with her may be the support she needs. Please get your friend some help.