How can you help a passive agressive best friend?

This is very common and very frustrating. I see it a lot in people. I don't know that there's anything you can do if she doesn't see that she is that way and want to change. Maybe you could sit her down and tell her as gently as possible that she does this - say that you are only telling her to try to help her. Then, ASK if she wants you to help her stop, maybe by having a code word or something you can use to let her know. In this first conversation, don't threaten to stop being her friend, but say that she will have a hard time making and keeping friends if she doesn't change. After that, if she continues the behavior, tell her calmly that you already have discussed the reasons why and that you can no longer be friends with someone that treats you and others badly when she is aware that she does it.