Games

For the record, I use a mouse with my non-dominant hand and I can play even fast paced FPS games like Titanfall competently enough. I actually used to dominate on Splitgate for a while. It's a skill that can be learned. I have the advantage of having done it my whole life and I fully acknowledge that's hard to replicate, but I think that with some practice anyone should be able to get to the point where they can play slower, primarily mouse driven games like turn based RPGs. Real time with pause might also be doable if you bind the pause button to the mouse (a mouse with some extra bindable keys would really help here). Anyway, just a thought.

If those options don't work, maybe look into games like Vampire Survivor, or Realm of The Mad God (though I think the latter does need some rapid mouse inputs when looting, so maybe not so good).

Super Mario Run was quite fun as a single-button Mario platformer. You could also try visual novels or point-and-click adventures, or even something just with simple controls like pokémon.

Caves of Qud is keyboard only option (no mouse), mostly just need wasd. Definitely an atypical RPG but very fun all the same.

I went through the same thing a while back

I played a lot of turn based RPGs

https://youtu.be/oRLmc6Gd628

The Final Fantasy pixel remasters are all old school turn based RPGs that can be played one handed on the steam deck. Certainly not all 50 games but UFO 50 has a bunch of games made for a non-existent console that are all pretty simple and I would bet a lot of them can be played one handed. Both Crypt of the Necrodancer and Cadence of Hyrule are rhythm dungeon delver games that probably work one handed too, technically so would the new game Rift of the Necrodancer but that requires more skill than I have

Oh, and for the RPG fan Disco Elysium is almost certainly up his alley and perfect for one handed play

Puyo Puyo Champions has a one-handed controller preset. D-pad or ABXY to move, shoulder buttons to rotate.

Only caveat is that if you want to play online, only the Switch version is actively populated, so I can't recommend the game on any other platform.

Dungeon crawl stone soup. Can be played with keyboard only.

Pinball with arrow keys mapped to left and right flippers?

Curious Expedition!

Not really helpful in terms of gifts as they are free to play mmo's and they use keyboard and mice but both have some script capabilities which allow for one to be able to play alternating between keyboard and mouse or not use mouse at all. Its the cryptic games champions online and star trek online. One thing you could do is play them yourself and learn the scripting and then gifting them useful scripts. with champions online turning the block to a toggle is the big script thing and with star trek online its making an emergency power to spacebar one (this is reference to a script where tapping space works through all the abilites put into a shorcut area that can be swapped out). really basic ones will dump everything into spacebar but more nuanced ones will have a key for low cooldown abilities and one for long cooldown offense and long cooldown defense. there really is no limit to the messing around.

deep rock survivor is a surprisingly fun and simple game that is designed for one handed play

If it's more permanent show them an Azeron Cyro at https://www.azeron.eu/ It's a mouse with a thumbstick, you can play almost any PC game with one hand. Comes in both right and left handed versions.

One Finger Death Punch and One Finger Death Punch 2

What more is there to say? The reason why that works perfectly is already in the title of the game.

Caution: this game is addictive

All the games. Every last one of them.

One handed controllers exist.

Atomic Heart

Rafa Nadal is right handed and plays left handed. If someone has issues with a non-dominant hand then it's time to start making the other one dominant.