Thanks all.
In answer to a couple of questions above:-
Yes I've explained my concern to the guy - more than once - he cheerfully acknowledges and then carries on as before.
I think that you're right about covering the instruments, I'll do that from now on and not take the covers off until the guy's got the hang of it. I did try that already, but he's persistent so I guess I should be too.
Yeah, the guy has spent far too much time on FlightSim, but claims to have given it up now. I'll be watching for the hairy palms and deteriorating eyesight though.