As the FI for the dual revalidation flight you are P1.
You have not done difference training for taildraggers.
You cannot fly it as P1
Since when has there been any "differences training" for taildraggers? They aways used to be part of the SEP class.
I am also fairly sure he can do this anyway. The purpose of the dual flight is not to give instruction, but simply to ensure that the flight is conducted safely. I've signed people off on dual check on taildraggers before - I've certainly never had any "differences training" on them.