I decided to tell a guidance counselor about my friend who cuts.. what happens now?

Your guidance counselor will handle the situation calmly. if she is any kind of counselor at all. I don't think the cops get called unless there is a suicide attempt. Your friends parents will be called. Alot will be up to them how to handle the situation. I hope your friends parents will help and not hinder (be part of the problem) and get her the physiological help she needs. She will be mad at you at first but will thank you all later. If her parents don't want to admit there is a problem and get her help the counselor may can involve the courts and get her help that way. Keep update on whats happening with her or not and keep pushing for her to receive help getting a hold of this compulsion.