How can i avoid being with my best friend's best friend?

theyve been freinds for 7 years, this girl (kelly) ever since i met her she seems shallow and a bad person, my best friend even told me that kelly made fat jokes about her, and kelly called me ugly once behind my back but idc cuz im my friend's friend not hers, i just tolerate her when she hangs out with us, idk why my friends still her friend maybe cuz they have history together idk its not my descision, but this girl is such a shallow person i feel like shell only be friends with pretty people when shes not that pretty she wants a nose job and she always brags about her big boobs bla bla, sometimes when my friend invites me to her house shes there too!! and its anoying cuz all we talk about is clothes and guys with her its so boring