Maybe it would help to talk to your parents about it. Also if you don't want to talk to them, it might help to talk to a teacher, youth leader, or the school nurse. Those people can all give you good advice about how to help your friend. Here is an link that might help too:
How Can I Help a Friend Who Cuts?
Sometimes when someone close to us is hurting themselves, it feels like we are betraying their trust if we tell someone. But sometimes, that is the best thing we can do for them. If you tell someone what she is doing, then she can get help so that she stops. Cutting is serious, and this is something she needs professional help with so that she is able to stop hurting herself. If you tell your parents/guardians, they can help you find the best way to help her. She might be angry that you told someone at first, but believe me, someday she will thank you for caring enough to speak up and get her help. I hope everything works out and your friend starts getting better soon!