This would depend on the distro you use. Most distros will require you to enable a non-free repository before you can install anything that isn't Foss or open source from the official repos. You could also use an FSF approved distro. Keep in mind, the FSF will only approve distros that don't include any non-free anything in the official repos. Besides that, you just have to know the licensing before you install it.

Soulseek and Torrent sites would be my suggestion. I'm able to seed 24/7 and would be willing to help if you want it.

Personally I would recommend Linux Mint. It's based on Ubuntu so any issues should be easy to find and fix online. It's very similar in terms of the actual desktop to Windows instead of being completely different like Pop!OS. You should also be able to completely avoid the command line as well though I do encourage you to have your friend learn at least some of the basics so he at least knows how to use it.

Not sure how to help with this but as a temporary workaround you can append an "&" after the command to make it run even after the terminal is closed. Should look like steam --restart &.