How to unlock a write-protected microSD card for use in a Nintendo Switch?

Check if he's using the microSD card with an adapter that has a physical write protection switch. They usually have a slider on the side. If the switch is moved to 'lock,' it would prevent writing.

When a flash device locks like that, it often indicates that it has failed. It's not too common for a new card to die suddenly, but it does happen sometimes. If that’s the case, you might need to consider getting a new card.

It sounds like the microSD might be in protection mode, which often means it's not functioning properly. Sometimes these cards can lock themselves for data recovery purposes when they fail. You could give formatting a shot with the tool from sdcard.org. Just start with a standard format and if that doesn't work, try a full overwrite. But honestly, if you manage to get it working, I wouldn't trust that card for important files going forward.