Yes, I too would wait - he'll be home soon, she can share that she is unhappy and why, say shes attracted to somebody else - he may not be AS threatened as its another woman, so its possible that can open the discussions more easily, and they could perhaps reconcile AND allow for a poly relationship, or at least end things without anybody feeling betrayed..
I'd caution my wife to hold off, you'll all feel better about it. Nothing is stopping them from having a date next week after things get more settled with the husband, one way or the other. It's not a healthy entrance into poly to cheat, and I'm guessing you'll feel conflicted enough about it from what you've said that it will affect you negatively as you move forward (after all, you came close to being in the other husband's position)- and that is something you need to be clear about with your wife.
Happiness will never come to those who fail to appreciate what they already have.