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.

FTL ftw

https://www.gog.com/en/game/faster_than_light

Papers please should be playable with one hand only and is great at burning time

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.

https://store.steampowered.com/app/2008920/Lorelei_and_the_Laser_Eyes/

Brilliant puzzle game

Designed to be played on a controller with one hand so you can take notes with your other hand.

Such a cool stylish game. Highly recommended.

One really handy thing with the Steam Deck is the ability to remap all of the buttons (as well as the two paddles on the back for each hand), so one could probably make a decent one-handed control setup for 99% of turn-based games. Even ones that require the use of the mouse, given the Deck's touchpads.

Vampire Survivors requires nothing except the stick/dpad outside of menus (and I'm pretty sure you can use the touchscreen for menus, too).

If your friend(s) are stuck using the dpad, it might not be suitable, but Crypt of the Necrodancer only requires four buttons or left, right, up, and down (and you can assign buttons for the button combos normally required to do things like use bombs). This assumes that they like rhythm games.

I remember that the dev of Legend of Grimrock added an option for movement with the mouse because a disabled gamer wanted to play. Maybe that second friend should check that and the 2nd game out.

Using a mouse with your non dominant hand is annoying…

Yeah, for PC they'll probably have to grit their teeth and power through the discomfort. An alternative is getting a hand in some old emulated games. Anything from the NES era can be mapped to 4 movements + 4 buttons (start, select, A, B), which on a keyboard could be WASD + Q, E, R and F. Dunno how to set something similar on a Deck.

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.

Crypt of the Necrodancer is great fun if they don't have it already.

https://youtu.be/vNvjdVJFW9A

Ben Heck made a couple of one handed controllers a few years ago. One of these controllers might open up more games than remapping buttons.

Most my time with Crypt of the Necrodancer was during a hand injury. It's of course a pretty intense game though, so it's not for everyone.

A Short Hike might be a solid pick. It's super chill, no stress, and you can get away with playing it mostly with just the left stick and a button or two — easy to remap if needed. Spiritfarer could also work, though it might take a little tweaking depending on how much movement they can handle.

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/

So left hand only games.. driving games should work(enjoying NFS heat, atm), turn based games and isometric RPGs (plenty of those, depends on preferences... Balatro, slay the spire, XCOM, disco Elysium, etc... I could make a list if needed).

Telltale games like the wolf among us and stuff

Vampire survivors and all the similar games (halls of torment, brotato, soulstone survivors, etc)

I went through the same thing a while back

I played a lot of turn based RPGs

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

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

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

I've been playing Jotunnslayer: Hordes of Hel recently on PC and despite being relatively action heavy it's 99% left hand (WASD + Q + space)

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.

thronefall is a fun little TD/RTS style game with a nice visual style and good rouglite-esque progression.

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?

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.

Pokemon Let's Go Pikachu/Eevee both support single hand mode.

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