I didn't dig really deep into it myself, but an acquaintance was using
overthewire.org/wargames/bandit as a way to teach someone basic Linux CLI skills. It's sort of like a capture-the-flag game, but it's very ground-up. It starts with teaching you how to SSH at all, if I recall correctly. Do it from a desktop Linux client, and they'll probably get the idea eventually. I think I only did a few levels before I got bored/went back to work. (If you do this, please let me know how it goes. I've been saving the link in case I need to teach someone Linux but don't know for sure how well it works).