bf female friend

So me and my bf been together for two years. He has this one chick friend who he became close to over the few months. We both have friends of the opposite sex but they text on fb every single day, it's to the point she double texts him if he doesn't reply. She's in a relationship and their text are plantonic . I just feel slightly disrespected and can't wrap my mind around it. I talk to him about it and he said she's just a friend and that she tells him about her relationship , I feel she is filling a void... lol I'm kind of sick of it . They don't hang out or anything ....Anybody had this experience ??