This happens all the time with used Xbox Ones - explanation (Off-Topic)

by Mid7night , Rocket BSCHSHCSHSHCCHGGH!!!!!!, Sunday, December 07, 2014, 23:26 (3641 days ago) @ GrizzNKev

He can, but only on his gamertag. The way it works is that any account can play the console owner's games on that console, assuming the owner has it set as their home console. The console owner can also sign in on any other console to play their own games, or that console owner's games.

Ok, I think I understand. And I can't "fix" it from my end? I don't want to mess up his download content, but I definitely don't have the guy's contact info anymore, I checked. I remember that he didn't have any kids though, so that's something, I guess.