An Xbox Adaptive Controller paired with the Logitech Adaptive kit can get him most of the way there. He would still need an analog stick replacement. You can find some great custom ones on etsy, like from SevenMileMountain

Mouth controllers are crazy expensive and usually sold out. https://www.quadstick.com/shop

Sadly there is no one boxed solution for it. And you'll probably end up spending over $250+ to get it all together. The disabled tax is brutal.

I have some experience using Tobii eye trackers [1] as a mouse input replacement. It integrates quite nicely with windows and allows playing anything mouse controlled that is not too hectic.

[1] https://gaming.tobii.com/product/eye-tracker-5/

I don't know if you can pick up an old dance mat controller on the cheap, or even if they'd interface with whatever gaming rig your friend has, but those feature face buttons and dpad as nice big squares you can "press" with your feet. Might be able to get some gaming done with that setup without breaking the bank.

Edit: searching around it seems like there's a good variety of modern dance mat controllers available. Some on the order of £10 if you're happy with a questionable quality aliexpress one, or more like £30-50 if you want something with more reputation. Could be a good solution if you want to avoid the ~$250 entry cost that other folks were talking about.

The XBox adaptive controller might be an option. It foes get a bit spendy when adding components, but it is meant to be super modular.

VoiceAttack can be used to freely create arbitrary voice commands with pretty good recognition accuracy. It can send KB&M or gamepad inputs and run scripts and various other features. I used to use it to supplement controls for Elite: Dangerous and it worked pretty well. I imagine it'd get tiresome for complex games but could be good for simpler games, and for other computer tasks. https://voiceattack.com/Default.aspx

There are tons of potential accessibility devices like specialized buttons and breath control etc, but they're usually expensive medical devices or custom builds. The other commenters have already mentioned the consumer products I'm aware of.

I'm curious about how he broke all of those bones!

I feel like if I were trying this myself I'd give an arcade style game pad a shot and try using it with my feet. No idea how well it'd work.

Not sure how easy these are to get yet but they look like a pretty solid win if you can find one. https://www.engadget.com/the-mouthpad-turns-your-tongue-into-a-mouse-for-your-phone-184541021.html