This girl who thinks i am her friend ha been talking behind my back?

so heres the deal thi girl used to be my bestfriend, she started getting into a lot of trouble so we drifted. lately we have been talking a bit more then usual she even asked me to hangout this summer so i said why not? and anyway i have this other friend now who i became close with and me and her have started partying together lately and stuff. but my friend who i drifted from told this girl that my new friend is going to become a sl*t like me since we BEstfriends now. i want to say something to her because shes talking to me like she didnt say this when I know she did .. what should i say? shes very mouthy i jut wanna confront her, nothing rude