There isn't that much difference. But I'd pick something in the top 2-3 most popular. That way (once you show them) they can easily google their problem and l

There isn't that much difference. But I'd pick something in the top 2-3 most popular. That way (once you show them) they can easily google their problem and likely find a web page and a youtube video to show them. I picked ubuntu LTS because it's popular, fairly straight forward, and supported for 5 years. Change can be confusing and it's nice to minimize it and have long enough so you can plan when to upgrade instead of having things randomly change every 6-9 months. Set autologin, get rid of icons you don't need, and set them up a default homepage and some useful links. I recommend showing them a webmail instead of a mail client which tends to be more complex. Go over basic safety (one password per site), never giving a credit card number out (or only to trusted sites over ssl). If there's a local LUG (linux user group) it might be worth mentioning. If they want my help I enable remote ssh (with passwords disabled) and put my key on it. That way I can fix things, see their screen, etc.