Those newfangled immutable distros might benefit them, since they're unlikely to want to or care about modification, and it's extra security.
Set up the web browser with a strong ad-blocker, maybe whitelisting YouTube since non-techy people are unlikely to know what to do if youtube throws up a stink about the adblocker.
Set up auto-updates maybe. For safety and such.
Otherwise make sure it looks like a dead ringer for whatever they are already used to (usually Windows) -- Same background, icons in the same places and looking similar if not the same, panels set up the same way, etc. -- This is easier with Cinnamon or KDE Plasma. Gnome... Even if you like it, it's too different, yanno? Unless they're coming from Mac, and even then it's not the same.
P.S.: Lovely background of a gay bird prince. <3