Right so theoretically if he doesn't get hit himself, and each round hits with an Invigorating power, there is no upper limit to the amount of Temp Hit Points he can have.
Yes, if the defender is fighting people with his melee range invigorating powers and never being hit in return, yes, he can have unlimited temps, and the DM is eligible for a brain transplant for never attacking the fighter or putting them up against critters that can't hit him at all.