I hate to say it, but there is no "right" or "wrong" with this unless you do not give her any choice. She gave you the choice to say "no, absolutely not" and instead of giving her an ultimatum like that, you two worked on it and discussed it.
People have to make their own choices and sometimes all we can do is be there for them to share in their joy or comfort them if they fall. When one has kids, they have to learn this, but it applies to all people and relationships. At some point, we have to stand back and let people make their own mistakes and their own successes.
One should always be thinking of ways to make a relationship "better" whether mono or poly and no matter what stage. Even when I was pursuing Elric, I was trying to figure out not only how this effect my relationship with Cajun, but how I can use the NRE I was feeling to boost our relationship.
If I may be bold and give one bit of advice for your wife. If she does go, she should be strong enough to at least tell the guy that she would rather be platonic, at least for the night, as she doesn't want to hurt him or be hurt by a rebound. (despite the fact that you two met/hooked up as a rebound) And if he respects her and cares enough about her and what she is going through, he may be able to curtail his lust...if that is what she truly wants.
If she does go tonight, do your best to find something to do, something that she may not have wanted to participate in or something that you know will distract you enough that you won't be lying awake all night.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.