I should say right away that I am no chemist so what I say shall be taken with a big load of salt. Quoting straight from Wikipedia:
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself.
Even if some of the Phenidone gets used up in the reaction, this is not a key feature of the reaction, and in fact we would all be a lot happier if the Phenidone would not get used up at all. Fact is that lots of silver ions get reduced per active molecule of Phenidone and fact is also that platinum catalysts in cars don't last forever.

But anyway, if Jerry kindly provided me with a link to his exchange with Pat, we might save this forum from a pointless reiteration of arguments that have already been beaten to death. Maybe real chemists have a more precise definition of catalysts that I am not aware of and that would not cover a reaction like reduction of light exposed silver grains with Ascorbate in the presence of traces of Phenidone.