FWIW, this IS a published adventure, namely Expedition to Castle Ravenloft, that I was running. It's also worth noting that I was running it because I don't have a lot of free time, and was running a published adventure not only because I wanted to, but also because I don't have the time out of game to prepare a homebrew. So, almost ANY work on the adventure that I have to do out of game is too much.
Also, in my opinion, it would have been impossible for his out of game knowledge to NOT effect the adventure.
The player with out of game knowledge would have had to make a choice: either make the right choice KNOWING that it was right, seeming like he's cheating because he's using his extra knowledge, or purposely make the WRONG choice just so he wouldn't seem like he was cheating. Either way, it changes the game for the other players, no?
The only other option would be to back his character out of the situation where he already knew the answer, and let the other players make the choice - but then he might as well not be playing the game. What's the point, then? Which, coincidentally, he isn't anymore, so the effect is the same.