Friends since they were tiny...

They've been friends for donkey's years and he's always sworn to you nothing's ever happened between them - he's never even thought about her in that way. OK, but does she feel the same way? Even if he only sees her as a friend, you can never be sure how she feels about him.   

Whether she likes him or not, she has an important place in his life and she'll want things to stay that way. They've probably been through lots together, from childhood and puberty to adulthood, and they've shared lots on the way: relationship problems, family problems, interests, nights out, and other friends.

The way she sees it, her close ties with him give her certain rights - one of which is the right to say exactly what she thinks about his girlfriend! She'll scrutinise you, everything you say and do, tell him all the bad things and all the gossip she's heard about you. And obviously she'll act like your new best friend when he's around, which will make him appreciate her efforts all the more! 

The problem lies in the fact that he values her opinions very highly (she's another woman, after all), and she could very easily plant seeds of doubt in his mind. He thinks of her as a sister and it'd never even enter his head that she'd deliberately put you down - she's his best mate, and can do no wrong in his eyes. 

The result is you end up hating her and resenting his lack of judgement. You either argue about her when you have to see her, or you leave him and her to it and make yourself scarce, which only gives her more opportunity to stick the knife in and convince him you're not the one for him...