My wife did EXACTLY this on the night of the 4th of July, bailing on me and the boys and missing our traditional blowing off of fireworks in our driveway. When she told me she was "going to a party with some girls from work," I looked her square in the eyes and said "I hope you're not going to be with (OM). That would be incredibly disrespectful to the family, especially on a night like tonite."
Of course, she lied to me, but at least I drew my boundary.
I think it flat out goes against your own sense of personal integrity to tell her to "go and have a good time" when you know she's with OM. C'mon, do you REALLY want her to have a good time with him???
You cannot control them, but you CAN let them know what your boundaries are, and -- if they cross them -- that you're going to have to take that into consideration in your own decision-making.
