You can put Gravity Badgers to one that would work instead of a maybe. It's definitely mouse only and I had a lot of fun playing it.

All of this week's Humble weekly also would be good. Point and click adventures are just that, point and click with your mouse and no need for a keyboard. Hidden object games also are pretty fun and mouse only. I think one of the bundles has Nightmares From The Deep: The Cursed Heart in it and that's one of the best of that genre. Mumbo Jumbo also has some good ones if you can get a hold of some keys from the Build a Mumbo Jumbo bundle

Match 3 puzzle games also are good and tend to be played with a mouse. I enjoyed 10,000,000, Dungeon Hearts, and Puzzle Quest 2 a lot, you probably can get Puzzle Quest and Puzzle Quest: Glactrix via a trade since they were on sale for cheap not too long ago. On a similar vein though it's match 4, Clickr is a lot of fun and also only uses a mouse, that was in a Bundlestars bundle not too long ago. Dungeon Hearts runs only $3 even if not on sale so that's a good one to grab and well worth it, was in some bundles, but it's been awhile.

Also, your friend should look into possibly getting a one handed controller. There's products out there for people like him that have the buttons on front and back so you only need one hand to work them. There's also some keypads as well intended for gaming on a PC. Get one of those and most games could easily become accessible again and they aren't hard to use at all. A quick google search found me a few potential products he could use.

I was going to tell you Cantine Deluxe. This game is so adictive.

Also, put ClickR.

Good luck and have a nice day :)

crusader kings 2 is mouse only if my memory is right

99 Spirits is nice, but your mileage may vary on how it controls with the mouse. Show him the demo to see if it's to his liking.
Toki Tori 2 can be controlled with one hand, but I don't remember if it's with the default bindings. Luckily, you can change them. (Clicky)
If your friend is into RTS, I personally recommend Dawn of War series.
Last but not least, Anomaly: Warzone Earth and its sequel.

Sam and Max

Life for Speed (realistic racing game) has amazing mouse controls!

I reckon you could play Dirt (arcade racing) just fine by mapping all controls to one thumbstick (requires controller :D).

Torchlight 2 (coop action RPG) should make no problems playing with only mousehand either :)

A mouse-only game that I am addicted to right now is Knights of Pen and Paper +1 Edition.
Its on Steam and most phone devices. Steam is mouse only, phones can be played one-handed (just tapping targets and stuff)

From my top games list; because there's not much point in recommending it if it won't be good! (in Alphabetical order):

• Civilization 5 - Same reason Civilization 4 is on your list.
• King's Bounty: Any game in the series - You may occasionally want to type a number, but it's mostly turn based and mouse-driven.
• SpaceChem - Puzzle Game. Using only a mouse might be a little slower, but that's nothing compared to how long you'll be thinking about the solution.

Free games!:

• Hearthstone
• Infinity Wars - Both are Card Games that are virtually 100% mouse driven; though the former is non-steam.
• Path of Exile - It's almost in the same boat as Diablo or Torchlight, where you would want a multi-button mouse to play; but you have so much flexibility in your character that you could focus on one or two abilities and still have a viable character.

walking dead

Most adventure games. Meaning games like: The Dig, Still Life, Monkey Island, and several hundreds of others. The list is huge.

Pretty much any RTS or Adventure game will do.

One Finger Death Punch is meant to be played with only one hand and one mouse.

It has been Greenlighted but is not available on Steam yet, it is available on Desura though.

most ARPG games can be played with the mouse only, if you have a good mouse, with multiple buttons. Torchlight 2 have everything on the screen, so you can click on every skill or option

• Knights of Pen and Paper +1;

• On The Rain-Slick Precipice of Darkness 1-4;

• To The Moon;

• Super Brothers:

• Sword & Sorcery;

• Titan Quest;

• and some of the Ys games.

• Cave Story+ should work as well, just rebind a few keys to use one hand.

• Also most Tell Tale games (Wolf Amongst Us, Walking Dead etc,).

-Might & Magic: Clash of Heroes.

-Fortix 1 and 2

-Sword & Sworcery

-Poker Night 1 & 2

-Point'n'click adventure games (Syberia, Broken Sword, Deponia, Gemini Rue... there's lot of them)

You could also co-op Brothers: A Tale of Two Sons with him. Normally you control each character with one hand, so it should work.

Warcraft 3?

Starcraft 2

Cookie Clicker!

Endless Space.

Triple Town

Super Sanctum TD

Perfection

Bridge It (Plus)

Papers, Please has a couple keyboard commands later on in the game, but I would say it's still 90%-95% mouse controlled.

Super Hexagon is also a great game and you can play it with just a mouse.

Sorry about your friend man, I wish them and you luck.

Fap is a good option XD I think that he could try FTL, RIFT, The Witcher 1, FEZ, Dota 2 and LoL

Ascii Grip v1 or v2 plus a USB adapter. Or another one-handed controller. Lot more options now than there used to be.

It seems you are pretty much covered about Steam titles. I'll add that you can browse kongregate.com and look for the tag "mouse only". You'll find a few decent games, and you can refine the research by genre.