Why do some friends tend to sort of backstab you?

about a year ago, me and my boyfriend lost our virginities to each other but then he dumped me two days later. he really made my life hell, and i had a friend who particularly helped me a lot because she knew him and she would just help me make fun of him and say all these things about him. they both hung out a lot over the summer because they have they same type of friends (stoners, smoker, druggies) and even though she specifically said that she would never go out with him (hes been trying to be with her even since we broke up, and she also dated his best friend) she is now currently dating her. and the worst part was that she didn't even tell me that they were gonna go out. now, i hate him, but shes my friend and i want to support her and i am kinda happy for them but it really bugged me that she didn't even tell me. why would she do that if she knew what he did to me? its funny though, because last year I was his valentine and now this year shes his valentine. i honestly do support their relationship, but i don't understand why she would be hypocritical why are people like that?