I think the best course would be to tell him something along the lines of “I’m sorry these games didn’t work out well for you and the experience didn’t turn out to be good for you, there’s still the option to dual-boot or try a different distro if you want but I understand if you don’t. Just know that these issues aren’t specifically because of Linux but rather poor support from the game’s devs, or more likely their publishers, games (about 90% of them) work fine through steam or Lutris unless the devs implement anti-cheats without linux compatibility so hopefully in the future if you happen to play more steam games you’d consider giving Linux another chance.” nonetheless I’d still say he should go on windows, find out that his games will likely still run like shit on there on his own and if he complains about it maybe bring up Linux again, gently and appropriately of course.