He has to somehow validate that his "friendship" with OW is worth destroying his marriage.
When pressed to end his affair or end his marriage, he throws the "control flag" up... pathetic a defence as it may be, its his default response.
That is the question(s) your FT needs to impress on him :
Is preserving a friendship worth destroying a marriage, a home, and an entire family? If it is just a friendship, your marriage should take precedence over that shouldn't it? You must understand sir that the more you try to keep control over this friendship to the detriment of your family the more convinced everyone becomes that you are indeed committing infidelity. I am also concerned about damage you may be doing to this woman's household as well. Have you spoken with her husband at all to determine how comfortable he is with you two being so close?
Your marriage should come before "buddies" night out... Shouldn't it?
I am still worried about this "everyone pretend he's not cheating so he will communicate" strategy.
His reputation seems terribly important to him. I still say blow it wide open so he has to own his cheating.
I realize you have done a fair bit of exposure, even to family members you hope would continue on your behalf. I am a bit out of the loop on how many people believe he's cheating and denounce his behaviour vs how many are just burying their heads in the sand or outright supporting him.
He seems earnestly determined to preserve his reputation.