HELP!!! WHAT CAN I DO?

ok so im best friends with a girl whos girflfriend recently broke up with her. Lately me and my bff's ex have been talking alot like during school and stuff.. She's really opening up to me about stuff but i dont know why because she never lets anyone in. I mean it makes me feel special and all but its killing my bff. my bff gets so jealous whenever me and her ex talk but her ex has been my friend too even before they started dating and i jjust think shes a cool person aand love taking to her. im not interested in dating her and im not replacing my bff with her either. The ex told me the other day that i was her closest friend that a girl (she pretty much is trans, like acts like guy dresses like a guy, etc.) but she says she doesnt like me as more than a friend. IDK though it really seems like she does. its killing my bff and i dont know what to do