There must have been a moment at the beginning, where we could have said no. Somehow we missed it. Well, we'll know better next time.
In the rules, they say that if a player have to play a red card but he doesnt have one in his hand, he can play any other cards. What happen if someone had a red card in his hand but did not play it for whatever reason? is it legal?
People with good memories would notice when they eventually play the red card. If it was an intentional cheat, and it's happened multiple times, personally I wouldn't play with that person again, at any game! If it was an accidental cheat one-time only, maybe you could just give a bunch of negative penalty points for cheating.