If he wants to expand his options and has a tablet there is a whole new selection of games that are basically made to only play with one hand, since tapping on a screen with both hands/fingers isn't very feasible...

Unfortunately I don't have a list of any RPG's because I normally don't play phone/tablet games, but when PSN went down over Christmas I decided to try out the Injustice game on my tablet and I actually really like it a lot. It has lite-RPG elements but is still a fighting game at heart. You collect "Cards" of the super-heroes/villains and they have an energy bar. Each fight costs energy and when you run out, you have to wait for the energy to recharge. However there are consumables that drop that allow you to instantly recharge your energy.

Also, when you have built up a team of different cards, each card has it's own energy bar so you can essentially play forever without spending any real money and getting a true sense of progression. Characters level up to 50 and have a Power and Health stat. Levels increase both. Cards can also be promoted by merging duplicates together, which further increases their stats, and can be promoted (7?) times. Cards also come in Bronze, Silver, Gold and Special colors, so the potential to have a massive array or characters to use is there.

You play with 3 card teams, so it's like a tag system for the fighting. The gameplay is literally tapping the screen to attack, holding two fingers on screen to block, and doing random quick-tap and quick-swipe movements to power up special moves after you've gained them. There is a "scenario" type mode called Battle, a Challenge mode every set amount of time which completion will award you with a rare card, and an Online Battle mode where you play bot-controlled PC teams.

If your friend doesn't like fighting games then this may not be for him, but this entire game can easily be played with two fingers on one hand, and while there is a pay-to-win aspect to it (you can purchase energy rechargables or the Credits ((currency of the game)) to purchase new card packs with real money) but it is absolutely optional and in no way necessary to have fun and progress.

I'm in love with it lol. It's AMAZING to be able to play a few matches while I'm waiting for raid to form or sitting in DPS queue. Either way, hope he finds what he likes and gets better soon!