So I got a problem

On the new SG there is an option, that the only ones who can enter, are people on your whitelist. So you should have done that, because even if he/she gets the link, he/she can't enter. Also thanks for the opportunity, but I suck at puzzles, BUT somehow I managed to figure out some in The Secret World already :D

but how do you know that someone else just didn't leak your giveaway?

Well then, try and contact support. They will decide if the reroll is valid or not, not us :)

Just blacklist him and make another GA. 1 not delivered ga isnt bad, and its done in a good cause. Or just tick delivered and give him some made up key. I would be harsh in this matter. It was private and he had no right to enter. but its your choice what to do :) Try to track the leak, look at comments and who is he friend with :)))
im evil, i know im bad

First, I would ask him how did he get the link to the giveaway, and if there is any proof of his contribution. Who knows, maybe he'll tell you the truth. THEN I would ask for a reroll.

It might take them few days. Just don't worry about it too much. Stress makes things worse :D And EvE will give you enough of stress :D