I have a friend who is really really nice to me but mean to other people. What should I do to stand up to her without ruining OUR good friendship?

I know this sounds wierd, but please don’t say I shouldn’t be friends with her at all because we play volleyball together and we need to get along. I end up spending most of my time with her. Her main problem is she doesn’t know how to forgive, so when a person does something mean to her, she will be ten times as cruel back. Welcome to the life of teenage girl. What can I do to persuade her to change her behavior without fireing up her temper?