Well, he shouldn't be using distilled water in the first place. If he has a specific reason for it, there are certain minerals he will have to be dosing into the water. I don't believe aquarium salt is one of them. But, it's worth a Google search.
Honestly, keeping aquarium salt out of the water on a regular basis is for the best. That way it can be used as a sort of medication if some ailment ever does pop up. And aquarium salt is not what causes saltwater. There are actually tons of different "salts" out there, just flip through a chemistry textbook <3