To me it doesn't matter if we are playing a friendly or playing a serious tournament, I would inform them immediately and help them rectify their mistake. Whether or not it's just for laughs or for major money, I'm not playing the game to pay for my next meal and therefore, there's no reason to not play the game on as fair and honorable a platform as possible. If the game dictates that information should be hidden from me, but a quirk of my opponent is feeding me that information anyway, then I would feel like an ass for taking advantage of that.
I'm not going to play stupid because i gained information that I shouldn't have though. If they end up showing me a card that lets me know what sort of deck they are playing, I can't help but use that information. But if I can help make sure they don't reveal more information, then I will. If it costs me the game, I didn't deserve to win in the first place.