Heh I would play it as the player is now PART of the hoard and is now the DRAGONS personal pet. The player was too persuasive lol.

And now the shenanigans is trying to get away from said dragon. And the rest of the group will now be dealing with a dragon that wants its shiny back in its hoard for the entire rest of the campaign.

Nat 20s can make for some GREAT monkey paw situations. Its really fun.