How do i tell my friend her boyfriend is mean?

tell her personally. that's the best option. true friends tell the truth even though it hurts. no matter what will happen at least you explained it to her. now if you can't do that, i guess you'll suffer. but if you did tell her and she isn't happy about what you said, it's not your problem anymore. it's her problem to deal. at least you told her what is really bothering you and your boyfriend. good luck!