Purchasing plug adapter for American plugs while in Paris

Have you checked with your hotel front desk? They usually have adapters left behind by other guests.

Or should he just try a tech store and buy a wall charger that will take a USB-C cord?

That would probably be easy to find. You may even be able to find one at a Monoprix.

If there's a gift shop or book shop in a train station, they often carry adapters.

I second what jeanm said. Ask the front desk of your hotel. Did this in Athens but I had lost my Apple Watch charging cord. The front desk had a box full of cords and plugs left behind by guests.

Many grocery stores and souvenir shops have them. Try a Monoprix.

Train station or FNAC

Basement of the BHV ( Bazaar Hôtel de Ville) department store has EVERYTHING. I’ve bought many there over the years.
Metro: Hotel de ville line 1 between Châtelet and Saint Paul ( Marais).

Or should he just try a tech store and buy a wall charger that will
take a USB-C cord?

This is an ideal solution. Buy two so you have them for future trips. So much easier than adapters for US gadgets.

Any store that sells souvenirs to tourists probably sells adapters as well.

I've seen them in pharmacies, supermarkets, camera stores, electronic stores, etc.

Thanks everyone. He bought one in the train station here in Brugge.