Don't you just love it when these lying shits let us go about our regular daily lives - making plans and futures with them and forging legal and financial bonds, etc. etc. - all so it can blow up in our faces through NO fault of our own?
Well, I'm the most vindictive bitch on the planet (I'm beginning to believe, anyway) and I'd go on that trip with him because why should YOU miss out on going to Mexico just because he's a big lying shithead?
He's no doubt already told his 'soulmate' that he's dreading going away with you, that he'll hate every single minute of it, that he won't so much as touch you in any intimate way at all, and that every single second he'll be thinking of nothing but her and will be miserable being stuck with you...blah blah blah.
That's what these lying fools always say to their OW when they go on vacation with their wives. It's as cliche as it gets - but it's right on Page #46 of the Cheater's Manual, Chapter 3 - "Lies to tell your OW about how much you hate your vacation with your wife." Pffft.
Therefore, making these two chuckle-heads miserable would be my number #1 goal in life. And that's why I would go on the vacation - just knowing she was crying into her beer every night because he's with ME in the tropics would be worth it to me. But again - I'm a bitch and that type of stuff is what I do.
Once you're down there, you have the choice of having him thrown into a Mexican Prison (which would be my vote) or holding him captive and making him talk. Tequila makes people do funny things and sometimes acts like truth serum, that's for sure. Letting his cell phone float in the toilet for the 20 or 30 minutes you're in the shower the first night you're there would probably also elicit a response from him (albeit not a good one). But, oh well. It was an accident.
Gosh and golly, I just realized that would ALSO cut him off from the outside world - especially from those sitting at home ALL alone - like the pitiful losers they ARE - waiting desperately for that feeble text message of love from afar, assuring them that they're still number #1. Now THAT would be a shame if he couldn't send that all-important text to her, now wouldn't it?
The last leg of my plan would be his total submission to mass amounts of tequila followed by a brutal question and answer period under a bare bulb while I blew cigar smoke in his face. Liquor tends to draw the truth out of people.
If neither of those options work for you, then I would make sure you have all your ducks in a row (evidence wise) and confront him with all the shock and awe you've got in you.
Oh - and make sure you WRITE DOWN on an index card everything you want to say or to ask him. These conversations get amazingly emotional and way off-track, so sometimes a card with a bulleted list will keep you on track as you navigate your way through the emotional chaos.
Good luck to you, Blue.