Scenario 1 - She is about to tell OM that it is over. You say F it and confront her. In shame (and she should have it), she flips, she NOW knows that YOU KNOW her secret. So in pain she runs back to OM. What have you gain here? Did you accomplish YOUR goals?
Scenario 2 - She does not tell OM it is over and keeps leading you on. You feel like crap knowing that your W if banging someone esle. You confront her either after the trip or before it and one of two things happens - she ends it or she doesn't. At this point you will need to determine what it is that YOU want to do.
Scenario 3 - You do nothing for a long time, while your W keeps banging someone else. You sit around hoping that she "wakes the F up" or finally sees how much you love her or she comes running back to you when OM dumps her ass. Not the ideal scenario but hey it is an option.
Scenario 4 - MIR has no objecdtive evidence to indicate his wife is about to end things with OM -- in fact, what he does know, is that they are about to get together (although we don't know for what purpose, but c'mon . . . ) So he gathers his evidence, and lovingly but firmly confronts his wife and lets her know that while he wants to remain married and work on their issues, he has no intention of doing so while there is another man in the picture. He tells her to "end your affair, and come back and work on the marriage with me, and I think you'll find me more than willing to work on all issues -- including my own."
It's naive at best, and dangerous at worst to operate on this assumption of "what if his wife is about to end it with OM?" when he has zero objective evidence of that.