What console do you have? I'm sure there's some way to uninstall it, but I've never done that myself.
Edit: Until there's a better solution, this DLC doesn't really change the game at all, if he doesn't want to play those missions. He just needs to avoid that 1 mission trigger for The Artifact. Right now he's still getting an additional island out of it: Isla De Los Annilos.
Uninstall the game to uninstall the DLC
At the moment there's no other way