My best friend twin brother problems help?

My best friend is twins fraternal boy girl n they r 15. The boy is a total ****** to her, for no reason! He calls her ugly n he always throws things at her. She doesnt have much friends, and the brother is half the reason she has low self esstem. He just so mean ! He once even called me a whore for no reason. Can i do anything to help? Should i tell the mOm? Idk if shes told the parents before. I tried stickin up for her but her brothrr kinda scares me 0.0 I just feel really bad for her!!