It’s a position no bachelor party attendee wants to find himself in:
A groom-to-be grapples with the existential consequences of marriage — up to and including the whole “last person I’ll ever sleep with” part — after a few too many Mind Erasers. Then he meets her: one last other person he can potentially sleep with. A situation ensues.
So what are your duties here? Well, that depends on a few things. Let’s review.
Is the groom in an open relationship?
If the answer is anything but a solid “Yes” that is recognized and has previously been acted upon by one or both parties, politely extricate him from the situation.
Has the groom been granted a proverbial “hall pass” by his bride-to-be?
If the answer is anything but a solid “Yes” (in which case, weird, but to each their own), same as above.
Is he so blotto he’s not even going to remember this conversation tomorrow?
Take him home, dummy. You’re not in college anymore.
Is he not completely blotto, and his desire to cheat potentially a harbinger for things to come?
Sit this guy down for some brass tacks. Is he sure he wants to get married? To her? At all? Has he had an affair before? Is he honest with his fiancée about his misgivings/past transgressions? The list of things you should never, ever, under any circumstances let a good friend do is short, but diving headlong into a doomed marriage is one of them. Oh, and wait until you get home to have this conversation, obviously.
What if he already did it?
You need to try to talk him into being honest with his betrothed about what happened. You do not need to tell her yourself, or anyone else who could start a game of telephone that she’s waiting at the end of. You need to emphasize to him that the sooner this conversation happens, the better. Assure him that relationships can survive this, but sometimes they don’t. Either way, he needs to come clean.
This isn’t life or death, but it’s pretty damn close.