There is no way he is telling the truth.
The chances of getting a key correctly by guessing is just impossible.
Highly improbable, but not impossible!

Sorry, had to do that.
Anyway, it sounds like he either put the keycode in wrong, got the wrong number, or something.
Or maybe it's an internal thing, it depends.
But you can't really hack a keycode unless you intercept/hack the email and/or email account that the keycode is in, which I have no idea how that works at all.