Can't he just send you a Kolmogorov-random file? The definition of randomness (in Kolmogorov sense) basically corresponds directly to his challenge.
Also, Kolmogorov-random sequences vastly outnumber non-random sequences in general, so with a long-enough file, I wonder how certain he can be that he has generated such a file.
http://en.wikipedia.org/wiki/Kolmogorov_complexity#Kolmogoro...