Games

For the friend who enjoys BG3, you can play Pathfinder: Wrath of the Righteous pretty much entirely with the mouse. The base game is 90% off on steam right now too.

Pathfinder has a massive learning curve compared to DnD 5e though, but that just means you can get an extra 50 hours of entertainment out of the game before you start the story, just by figuring out all the options when creating your character lol

Edit: whoops, seems like the steam spring sale just ended. The game is still absolutely worth it at full price though

Just FYI, there are a number of controllers out there for people who only have the use of one hand that are relatively cheap. I don't always have the use of both hands, and I can definitely say that it's possible to learn to use regular controllers effectively one handed as well. Both the Logitech F710 and the 8Bitdo Pro 2 work really well in my opinion. People have laughed at me, but the 8bitdo Zero 2 surprisingly works really well too, but is much more limited for inputs and thus, more suited to retro games.

https://youtu.be/oRLmc6Gd628

If this is a long term thing, you might consider a left handed gaming mouse with programmable side keys.

I hate to recommend Razer cuz their build quality has taken an absolute nose dive, but the only one I know of is the Razer Naga: https://www.razer.com/gaming-mice/razer-naga-left-handed-edition/rz01-03410100-r3m1

Setting some of the side keys as WASD and the usual surrounding support keys (Q, E, R F, G, X, C) and you can play almost any game one-handed.

The learning curve to get your brain to accept mouse movement and clicks with your left hand is annoying as fuck though.

Pinball with arrow keys mapped to left and right flippers?

Curious Expedition!

All the games. Every last one of them.

One handed controllers exist.

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

XCom and XCom 2 can be played entirely with the mouse. Minor typing if you want to name your soldiers, but nothing requires quick reflexes. Everything is turn based.

There's an older breakout style game on steam called Shatter that can be played entirely with the mouse and has a banger soundtrack and neat visual style.

Emulation opens up a lot of options for old school turn based games. RPGs, turn based strategy. Any of the Pokemon games gen 1-3 can be played one handed with some clever button mapping. Any game made to use just the Wii-mote as a pointer would also work, but I don't know those off the top of my head.

You might want to look into one handed controllers, or something like the FLIR USB dongle that you can use to map IR TV remote signals to keyboard button presses. Just need to use a remote that doesn't already control something.

Atomic Heart

I don't understand. You have two friends, each with a good arm, that's two good arms. Tape your friends together and have them coordinate and BAM: two-handed gaming.

Have they tried narrative games like the ones from Telltale, Dontnod, ... ?

You might want to check out this post : https://jlai.lu/post/16859633 Text is in French, but the picture shows a collection of 69 one-handed games on steam

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).

I think you should be able to play Organ Trail one-handed. It's a zombie apocalypse version of Oregon Trail.

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.

Balatro
Slay the Spire
Brotato
Vampire Survivor & Clones

You might be interested in the videos made by Champutee, a one-armed gamer who has done several experiments with both genres and controllers to continue enjoying gaming.

I guess games like Monkey Island (1 to 3, and Return, the 3D ones would be harder) could work using the touch screen or touchpad working as a mouse

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.

I think a fair amount of survivor games like Vampire Survivors and 20 minutes til dawn are intended for just one hand. Then there's every turn based strat game like Civilization and stellaris, along with every turn based RPG out thete

Here's a curator for one handed gaming as well. https://store.steampowered.com/curator/41112101-One-Handed-Gamer/

If they have a keyboard and are into turn-based role playing games then I can recommend Caves of Qud. It has a bit of a learning curve but once you get the hang of it starts to open up and it's just incredible!

Balatro is turn-based and is easy enough to control. I myself play on mobile, but Steam Deck shouldn't be very different.