It had better not be something that requires decompiling.. First off, I don't think doing so is even legal. Next, that's awfully mean because we'd have to learn the game code despite everything having names like int1, string2, char5, float10, etc.
And of course there's always the potential that he had a brain fart and stuck in in the code as a comment or an unused string lol which means that it isn't in there. Honestly, unless they're programming in assembly (which I really doubt), even the structure of the code is going to be quite different because modern compilers pull all sorts of magic when making making an executable.
The above means that you aren't going to have any dead-end strings as the compiler would simply rip them out (not to mention I've already searched the executable for this sort of thing and while it's possible I missed it, I did check). Really it honestly seems like the only options remaining are things hidden in the game itself..
Of course, I suppose they could manually write a block of assembly code that contains the secret.. This is probably the most likely scenario because I don't think compilers touch assembly code with their magic but I suppose I could be mistaken.