Honestly , the only real danger filtered well water should afford is bacteria or spores. City water will have trace amounts of chemicals. Both will have dissolved minerals, but that's it. The risks are minor at best.

The thing with distilled water is consistency and a negligible price. It costs you practically nothing to make the previous risks non-existant. Some may consider it a pain to store and use a jug instead of the tap.

Both arguments are valid, so I say just let him do what he want's and mind your own business. Give him your opinion once and just let him be.