I'm tired of her , is she my real friend or not ?

Well I am new at highskl and everything I have 2 best friends in my cl and there is this other girl , one of my best friends is ill so she hasn't come into skl for 3 days now so it was me nd my other bff , but in our cl there is another girl and she hangs out with us aswell but since my other friend has been ill , the girl hasn't hung out with us . She hangs out with some random ppl in my cl . And its annoying coz since my other friend has been ill she has gone to hang out with another group . she says shes still our bff but she is even gonna go out shopping with the other group , I need there to be a way to get her bak but she likes the others now . PLZ helpppp !!