go through their list of games on protondb to see if linux would be a good fit, it might not be, depending on their library.
you've got a nice 'short list' of distributions to evaluate. i'd recommend setting each up yourself on a spare system to see how 'point and click' they are in setting-up, running, and updating the games they play.
also keep in mind the more hoop-jumping and manual configurations you need to do, the greater the chance of something breaking--like during os or wine-related updates. those kinds of issues are the ones your friend will be relying on you to fix for them so you definitely want to minimize the chances of that happening.
absolutely. some cpu can draw more than the 180 watts of a 5060ti, even without actual 'overclocking', all by themselves when boosting their own clock (such as intel's "turbo power"). even a '65 watt' ultra 7 265 can get up to that.