Tackling bullying

My friends sister (about 15 years old) is getting bullied at school. He has made his female friends speak to her at different points and give her various perspectives. Could you suggest ways to help build her confidence and personally work through the bullying? Either books or songs or any tactics that I am missing. Parents have spoken to school etc but how do we equip kids these days? When we were younger, we did not get that sort of open support but building a confident woman also takes a village! I am a parent myself.