Lynn is definitely an immature "mean girl" and you're right to avoid her. I couldn't tell if you still see Julie sometimes without Lynn around. If you enjoy spending time with Julie then do so, and if you think she should have shown more loyalty then don't hang out with her. I would probably stop being Julie's shoulder to cry on if she still comes to you for that. It sounds like you gave more than you got so if you feel like the friendship has run its course then move on without looking back.