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

Divinity: Original Sin 2 is basically just more BG3 (though with less production quality) so that should be right up your RPG-loving friend's alley. Turn based combat and click-to-move lets you play the whole thing with just your mouse I'm pretty sure.

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!

Curious Expedition!

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

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