Linux

As a gamer myself I don't think it's appropriate to try to deprive someone of a hobby that they genuinely enjoy. As a long-time Linux user I stick to games that work well on Linux, like Overwatch. But unfortunately games aren't interchangeable - if League is your thing, Dota might not be an acceptable substitute.

Better to argue for dual booting than to try to cut off a friend's personal relationship with their game.