Instead of telling her what to do and then expecting her to apply it in real life with no experience, do a little bit of role-playing with her. Oftentimes I play the mean girl and then let my daughter test out the responses we've talked about. At the counsel of a professional, I have advised my daughter to simply ask her gal pals, "Why are you being so mean to me?" We practice this often and since doing so, I've witnessed her using the statement on her own. As they say, practice makes perfect, and role-playing is a great application for dealing with mean girls.
Source: Flickr user Daniel Bentley