Steam-to-Switch license carry-over? Update: Nope

No, it's 2 different stores/ecosystems.
But you can buy it on steam for cheap as this game was thousand times in bundles

Just look @ price history here
https://steamdb.info/app/21130/
&
here
https://steamdb.info/app/204120/

There is no legal way to play games from your Steam library on your Nintendo Switch.
A few explanations would be: he has a first gen Switch and hacked it to install android, she may have meant a Steam Deck instead of a Switch, or yes, he could have bought another copy.

Occam's razor suggests that a likely explanation would be a miscommunication or misunderstanding somewhere along the line.

My bet is on the "Switch" actually being a Steam Deck.

Oh, be a good uncle and just buy him a copy.

It turns out that the lad saw the game in the Switch Store and downloaded it not realizing that this meant spending his parents' money.

Thanks all, for helping with the mystery!

He says he prefers it on Switch.
One problem is that he didn't know to use the mouse to turn the perspective.
Another aspect is that he already knows how to get a Switch game onto the livingroom television.
And finally, he says on Switch the game is multiplayer, as if the Steam version isn't.

I know on Steam it does local co-op and remote play together, but I'll have to figure those out for myself before I can properly explain them to him.

The first medium is usually the favorite in many fields (likely due to the resistance to a new learning curve).