Ah see to me the usability of the established filter seems much more related to the amount of organic inputs than to the tank surrounding it.
If he uses his established sand and rock and only adds what he has now as far as organic/nutrient pollution goes (no increase in livestock, pre-cycled dry rock, etc.) then it would seem the filter would be at the same level in the new tank as it is in the current tank. No?
As I said before big difference would be if he added stuff that contained additional organic matter/nutrients such as decaying new liverock, etc.