Boxed Card Question

[Dante1126]

I'm a TD for a local small poker league. For the first time tonight I had a boxed card situation come up, but with a slight wrinkle.

Players deal for themselves, there's no professional dealer. After all pre-flop betting was finished the dealer proceded to deal a flop. However, the third card of the flop was boxed (turned face up). The players called me over to make a rulings, and to my understanding any boxed card found if treated as a meaningless scrap of paper and set aside. Some quick research at home after the game also confirms this.

The problem though, is one older player insisted that the card should have still been live because it would have been part of the exposed flop anyway; and another player staunchly agreed (although the this player isn't particularly bright, and the live card would have given him a flopped set...)

Any thoughts, experiences like this? I can't find a single exception to the boxed card rule.