Well, the D-Disk should appear at the northern end of the table in the room you fought Gonzap at. It doesn't actually appear as one of those red/white, "Pokéball chests" it just looks like a twinkling light on the table, and you have to walk up to it and press the A Button to pick it up. Maybe your friend just overlooked it or something. The Disk should have appeared after beating Gonzap. Perhaps your friend hasen't beaten the trainers with the Shadow Smeargle and the Shadow Ursaring in the Snagem Hideout yet. I don't know if that would have an effect, but it could be worth a try.