Thats horrible!! I'm so sorry that happened to her! I wish I had some good advice. As a mom I am sure you want to do your best to take care of her and I think you are by listening too her. The best thing I think for her to know is she did nothing wrong and should not feel ashamed. Eventually other kids are going to realize these boys are bullies and will have nothing to do with them. Chances are they have done other things to other people to make themselves look "cool" which will eventually catch up too them
I really wish I had some better advice but I think you are doing a great job as a mother just by listening, and the fact she came to you is such a big deal at that age. Alot of kids that age would not tell a soul and lock themselves in their bedroom.
HUGS
Cher