Am i backstabbing my friend and betraying her trust?

okay so my close friend hates this girl and the girl hates her aswell (this girl was my friend till my close friend told her about me so i stopped being her friend) but now recently this girl and i are talking again and i feel like im doing something wrong my close friend doesnt know that were talking again (btw were not like really friends just talking alittle) I wanna know if this is backstabbing my friend and betraying her trust i told her i wouldnt talk to this person because she tried to use me etc etc but now she says she changed and wants to be "friends" but i have a hard time deciding do i just become her friend or tell her off please answer