I think sara makes a great point. Christmas is coming, the goose is getting fat, who is your accomplice going to be with on the 25th? You, or the mother of his children?
The fact they are not married is neither here nor there.
Look, I've been there - before my current partner (we are not married) came along, I was bedding a married woman, so I'm far from a moral beacon, but one day, whilst walking my (now sadly dead) dog, I happened to see her in the park with her husband and kids and I was absolutely consumed with guilt and finished it the next day.
Seriously, move on. It'll be doing him a favour as well - none of this "I've got a meeting in [wherever]" pony that he is, without a single shadow of a doubt, trotting out to his Mrs.