Sorry to be so vocal, here, I feel empassioned for some reason. I think everyone has a bit of a 'mean-girl' in them...and that really does seem to come out the worst in adolescence.
Here's what I teach my teens:
There's different ways to handle conflict, and different ways are warranted by the situation at hand.
1) Aggressive: actively trying to deal with conflict in a hurtful way (whether emotional or physical).
2) Passive: doing nothing (or avoiding) in the face of conflict.
3) Passive-Agressive: handling conflict by attempting to hurt someone in an underhanded way, so that you are saying one thing but implying another by your actions or expressions.
4) Assertive: actively attempting to solve your side of a conflict without knowfully causing harm (either mentally of physically).
The best remedy to passive-aggression is assertiveness. But this is sometimes extraordinarily hard to teens (and adults) to master. That is where the 'comeback' coaching I talked about earlier comes in. I don't mean that you help your dd think of nasty things to say to her friend. But maybe, if your dd feels like her friend is getting really irritated with her recent great grade or academic success, she could say, "It seems like you're mad that I got an A". The other girl may not agree (most likely will not)...but asserting oneself and calling attention to the underhand stuff often shuts it down.
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