I want to be her friend, but I don't want to run into my ex! What do I do?

She is dating my ex's best friend/roommate. When I invite her out she wants her bf to come. Then she invites me out with her and him. I don't want to run into my ex. I also dnt want his nosey best friend going back and telling him stuff if I bring another guy along or if I'm out w/ them & another guy tries to talk to me. Should me & her just not be friends?