your best freind wedding

You are at your best friend’s wedding just an hour before the ceremony is to start. Earlier that day, you came across definitive proof that your best friend’s spouse-to-be is having an affair with the best man/maid of honor, and you catch them sneaking out of a room together looking disheveled. If you tell your friend about the affair, their day will be ruined, but you don’t want them to marry a cheater. What do you do?