relationship issues

I hate to be that person that comes on complaining about there relationship
But my boyfriend has a best friend who's a girl and she hates my guts. She's really mean to my boyfriend and always makes him feel shit about himself, but is nice when she needs someone to talk to. I hate how she treats him but he lets her and he says he loves her cuz she's her best friend. 
I'm so annoyed that he lets her treat him that way but I have no idea how to confront him
Any advice