I am buying a friend’s PC for games. I want to avoid windows if at all possible, and I’m wondering what people’s experience using Proton in Linux for gaming has been. Are there certain publishers who use libraries Proton doesn’t handle well? Are there distros to avoid using with proton? Any other notes I should be aware of?
Others have already answered the questions you asked, so I’m going to answer the one you didn’t. If your friend’s PC has an NVIDIA you will need to use the proprietary drivers to get the best performance. This also means that you probably should avoid distros that use Wayland as default since NVIDIA can be a bit hit or miss there.
As for distro all of them will play nice with Proton, but you don’t seem to be very Linux savvy so I would also stay away from complicated distros such as Arch or Gentoo (or their derivates) which assume the user knows their way around. I used to recommend Kubuntu, but recently I read a post about a guy complaining because his PC was thermal throttling under Kubuntu. So my recommendation would be either Pop_OS! or Linux Mint, both are very beginner friendly and should work mostly out of the box. The more you know about Linux, the less difference the distro makes, but you don’t want to jump to the deep end without knowing how to swim first.