The no contact letter needs to happen, but I think it should be presented to her more like, "I am ready to work on our marriage, but I cannot do that unless the door to the OM is completely closed." Firm but loving, you know?
Tarheel has done that. 3x, by my count. His wife has given him her (non-) answer. For Tar to continue to restate his boundary would only serve to WEAKEN it.
I wouldn't much care how anyone would perceive me canceling MC. If she's unwilling to kick the 3rd person out of her marriage that she's unilaterally invited into it, and take the steps necessary to work on the marriage with Tarheel, then the MC sessions are a charade and a waste of time at best, and are going to be used as a platform for her to exit the marriage at worst. Either way, I wouldn't be much interested.