Tyrian 2000

And it's free.

Most strictly turn-based games (for example, Dragon Wars, Wasteland, the Gold Box games, Civilization and similar games, true roguelikes).

VVVVVV (you only need 3 keys for active gameplay; left, right and flip; anything else is not needed at any point where quick action is needed).

Any game that halfcoordinated plays on twitch or speedruns at a GDQ, though you might need to customize the controls. Speaking of which, if you're looking for games you can play one-handed, definitely look for games with rebindable controls; you can just rebind all the important controls so they can be reached with one hand.

There is plenty of games that can be played,one handed as I do it all the time.Even just typing this is using one finger.

If you're looking for free games, Ultima 4 will actually work. You do need the whole keyboard for the game, but as the game is turn-based, you can take your time entering commands and not be penalized for it; you'll just play slower than an abled person would. (In other words, I wouldn't speedrun this game one handed, but it's perfect for casual play.)

Speaking of Ultima, 5 and 6 are also manageable one handed for the same reasons. I wouldn't suggest Ultima 3, however, because if you don't enter a command for 5 seconds, you skip your turn.

Most adventure games can be played with one hand.
https://www.gog.com/games/adventure?sort=popularity&page=1

Huniepop. Great for stress relief, too. ;P

As was already said most adventures and turn based games should work just fine with one hand.

If your friend is more into action then I can definitly recommend A real shame that isn't on GOG. :-(

Had to quote the link because of the serial downreppers. But wouldn't that require two hands? ;-)

Trust me, you need two hands for that. :P

You know... nevermind.........

Of course, this doesn't apply to adventure/other genre hybrids, unless the other genre aspect of the game is one-handed friendly. For instance, playing La-Mulana (adventure/metroidvania hybrid) one handed might be difficult, whiie Dragon Wars would still work well. (I don't know about Quest for Glory; maybe I'll ask about it.)


Another idea:

If you have a mouse with an extra button that can be remapped, you could play Baldur's Gate and similar games; just remap one of the mouse buttons to the spacebar so that you can pause.

Alternatively, even without such a mouse button, keep your hand by the spacebar, and when you need to do something with the mouse, pause the game first.

Most point and click adventures. I like it because it lets me hold a cup of tea in my spare hand.

Most games that don't require a second hand for WASD movement can be played with one hand. Depending how many buttons his mouse has, it is very viable to play games such as StarCraft, Diablo or even League of Legends. Back when I still played LoL, some of my friends always had a hand free for smoking while playing :D

Besides point and click adventure games one hand friendly are usually turn-based strategies and RPG's. With a little effort you can also play RTS.