NC can be done still in these situations. It's like the 180 when you have NC with a WS that is still in the fog, you only talk about children and finances but that's it. In this case, communication with ex is only limited to the child and welfare of child.
You can set up requirements on how this will transpire. For example, you should be allowed to listen to the conversation or you have to be present when he talks to her or something so that you know he is complying with NC. IMHO the child should not be punished and loose a parent because of the parent's issues. This way the child is taken cared of but there is no "sweet" talk between your spouse and the OW.
Easy for me to say as this is a hard situation nonetheless. It's just a suggestion, I'm sure others here can chime in with better ones. Hugs your way.Love is a choice.
True love is harder to come by than soul mates. True love requires work.
Ignorance can be cured with knowledge. There is no cure for being an idiot.