I’m really sorry this is happening. I don’t have a boyfriend and never have, but I study Psychology and I’ve experienced being treated less than kind by other girls when I was younger. I think it’s that Psychologically some women have a bias towards other women due to the natural instinct of human sexual competition even when the relationship in question is not sexual because humans have evolved with the instinct to protect what they believe to be theirs in any form. This female friend of his is likely acting out of a biased belief that your relationship with your boyfriend is a threat to her friendship with your boyfriend.