Totally agree with leon. I've done a bit of observing with people who were completely unfamiliar with telescopes, and none of them had trouble using the focuser after an introduction that took, what, 5 seconds?
"Everybody is different. Turn here to focus. You'll see when it's sharp". I make sure I use the word "sharp". The hard part is usually finding the EP in the dark. Using the focuser gives a better feel for the instrument, plus it illustrates why it's best not touched once observing is underway. Also I never assume my focus settings will suit any other person. FAIK, they, or I, may need glasses but don't know it yet.