I have been told by more than one transmission builder that if a car has had the fluid flushed at regular intervals, that it would be ok to do so. But if the car has never had a flush, and has over 100k/8-10 years on it, introducing that much fresh fluid can damage the transmission due to the detergents in the fluid breaking down the old seals. The best route would be to take would be a pan drop with fresh filter, and top off with fresh fluid. If that does not fix it, it is time for a rebuild. So his story is very possible.