HELP NEED ADVICE!!!

Okay so this has been going on for months now. me and my boyfriend are very close like best friends. my girlfriend (girl best friend) we used to be super close but she started being weird and off and she’s so rude to my boyfriend she calls him a fucking loser , ugly , stupid , disgusting, dumb also just laughs at him because he gets embarrassed very easily. my boyfriend have issues with being happy and she’s making it harder for him.. he’s very shy and he doesn’t say anything rude or inappropriate to her? i don’t know what to do? i’m so scared to tel her off... i don’t want to hangout with her anymore and i dont like being around her because she kills my good mood... i really need the advice!!!