Oh dear.....let the games begin!!!
It's so hard for kids to find the place where they fit and and feel comfortable. Remember that this is a learning experience for her hon.
Maybe sitting down and having the talk with her about right from wrong (gently reminding her) and asking her how she would feel if she was treated the way she is treating her other friend.
May I be so bold as to make a suggestion?? When speaking with her, instead of saying you are disappointed in her, maybe you could say that you are disappointed in her actions and choices she has made. That will help her understand that no matter what she does you still love her and don't think of her as a failure, but that she could do better to make better choices. Sometimes it's all in how it's worded that makes the most impact, positive or negative.
Just a thought for you as I know how fragile young girls minds are.
Wishing you well m'dear

sabby