Just to pile on...

I guess that explains all the mysterious fires at observatories - they were looking at the moon and the intensely concentrated moonlight instantly vaporized the observor's eyball/film plate/CCD into an incandescent plasma ball...

For sure, he is full of it. It could be done (maybe), but you are correct about the immense size of the concentrating mechanism required (lens or reflector). It's all about the energy flux - the moon simply reflects some of the sunlight (not even the full spectrum - most of the short wavelength stuff is absorbed and re-radiated as long wavelength which does not penetrate our atmosphere) AND the energy from that reflection is insignificant compared to direct insolation. If you really want to debunk the guy, go dig up the energy flux in moonlight and compare it at the same location with insolation. I'm going to hazard a guess here and say that moonlight has less than minus 6 orders of magnitude the energy of sunlight (one millionth) - but you don't need to guess, you can nail him with the facts of the matter.

Or, there is a simpler way - ask him to show you. Give him a 30 minute headstart and then squat down beside him with your ferrocerrium rod and produce fire in 15 seconds...

Have fun with the expert!

Tom