Your school doesn't have a policy on FB friending? My place of work has a policy regarding us "friending" patients/former patients.
I think it is a good idea to notify the administrator. You might need some direction on how to proceed with something that happens outside of school hours. Maybe there is some precedent in your district?
You talked to the student's mother, which is a good thing to do. You could address this issue with all of your cheerleaders. Drinking is a common problem, and some parents think it is o.k. and normal for teenagers to drink.
I agree that the squad could be a positive influence for her. You coudl talk to her, but you can't save her. She may choose to go down this path whether or not you try to help her.