Just repeatedly tell her she has something on her face. She'll go look in the mirror and realize it's hair. She'll be really embarrassed but at least she'll think you just thought she had something on her face.
ok next time she mentions being in an argument with a friend or something just create a fake facebook account and berate her on her wall specifically mentioning it. She'll think it's a petty retaliation from that girl because girls can be cunts but she'll still be embarrassed and fix it.