this girl is creating way too much drama. tell her you will not talk to her while she is seeing someone period. then do not reach out to her and do not respond to any calls or texts from her. if at a point they break up maybe she will contact you or you can reach back out but staying in contact while she’s seeing someone will drive you absolutely crazy, make it impossible to move on, and give her her cake and allow her to eat it too. you will be helping her get over you.