Is it really OK with you if he talks to her? If so, then what is the problem? If not, then why did you tell him it was?
It is time to quit trying to control H's actions and control your own. What is your boundary here? Are you willing to piece while he is in contact with OW or not?
It sounds to me as though you had already set a boundary here. I'm not sure why you did not enforce it, but you need to make a clear decision in your own mind about what your actions will be if he is in contact with OW.
It is very unhealthy for both of them to continue in the R in any way. It is hugely unhealthy for sick OW to rely on him for support. The current situation isn't good for anyone. Why do you want to be part of creating and sustaining it?
One solution is suggested by ROOT. Either OW if a friend to both of you and a friend to your M, or she has no place in either of your lives. Given the history, if she is to be a friend to both of you, ALL communication needs to be entirely transparent.
But, personally, I see little point tolerating a continued R with OW.
On the other hand, no one makes it through piecing without the WAS repeatedly relapsing in terms of contact with OP. So, you need to ve very sure about how much you will tolerate and how much you will not.
