I'm with Shaun, just don't ever give a loaner to a backpacker. LOL!
I can understand why the guy was a little tweaked though, since it was a loaner. Obviously you felt comfortable with the guy's abilities, but idiot proof can be a relative term.
I've learned that no matter my initial perception, I need time to evaluate to be sure that the perception has merit. How much time depends on what I see as I go, progress from easy tasks to more complex ones can be a quick one, but I've seen guys plateau fairly early on and then can’t figure out why they don’t get to do anything else.
Without being there I can't say you were wrong. After understanding a few basics, I rented a 252 and went to work, but I've got an excellent background for it.
I wouldn't have given it to a first day guy and left, but I might have discussed, demonstrated, and then handed it over while I stood off to the side. But I'm not in your shoes. I guess the best questions you might ask are:
Would you have done the same if it was your machine?
And maybe more important to understand the reaction:
Would you be happy if someone else did the same thing with your machine?
Understanding that the stumper owner didn't have the same confidence is the key here I think. Had he been there, he might have given the same guy the keys. It's not that he doesn't trust your judgment, but it's the not knowing for himself that got him excited, I think. JMHO.