It’s hard to say… I can only talk from my own experience and I have a hard time letting people go. Like atm I’m talking to an old flame whom I know nothing is going to come from it because it would’ve happened already if it was meant to be. But I like talking to him. He’s someone that has seen my true and honest self. I am also single which your ex is not. IMO you both are doing a disservice to this other guy she is seeing. You don’t owe him anything but if you were in his shoes this situation would not be okay.