Imo, if one is talking divorce for many other valid reasons, then I would think there is no need to mention AP and just stay focused on those other valid reasons.
I would hope that the person seeking divorce would understand how having an affair will amplify marital problems and spousal quirkiness and how it has made for their own disconnection from the marriage with neglect on their part and that an affair that has not stood the test of time...in the light of day...might not live up to their expectations and might leave them wondering why they gave up a marriage that could have been worth saving by giving it one's full attention without an affair partner in the wings.
Clarity
"Once you say you're going to settle for second, that's what happens to you in life." John F. Kennedy

