Wanting a firearm to protect oneself is understandable, wheelchair or not.
Since your friend is considering a gun, the first thing I would do is get him a copy of Massad Ayoob's book, "
In The Gravest Extreme. After he reads that, then I would have him take a basic "safety" class. Hopefully he will get the opportunity to talk with a few people about different guns, and shoot a few. That will help him better decide.
Then I would proceed to a range where he could rent different guns and have him try them on for size. With all that said, the simplest defensive handgun to operate is a Double Action Revolver, of which I am a fan of. The negative to the revolver is of course the limited capacity, and the amount of time it takes to reload. Those issues can be dealth with through training and tactics.
I've come to realize that people that start on revolvers seem to more aware of the limited ammo capacity and tend to make their shots count better. Heck, even my groups are tighter with a revolver than with my Glock 19. If you can really learn to "run" a wheelie, it's a piece of cake to transition over to an autoloader. The same cannot be said in reverse, IME.
Also, since this is a first gun, if he goes with a revolver, I'd suggest a .357 Magnum even if he loads it with 38 Special loads. In fact I would urge him to use 38 Specials until he got better. If he goes with an autoloader I would urge him to go with a 9mm. Later on down the road he can look at moving up in caliber. What he needs right now is "trigger time", and that can get expensive.
Another thing to consider is some type of sighting device, no matter the gun. I'm a fan of the Crimson Trace Laser Grips for defensive usage. My current EDC gun is a High Power in .40 S&W with Night Sights and Crimson Trace Laser Grips. If I could only have one, it would be the Laser Grips.
Having, carrying or using a gun for self defense takes, IMO, a lot of deep forethought and isn't for the faint of heart. Too many people don't think about what happens after using their gun. Be prepared for that before you have to use your gun. That's why I say, if you really want to be a friend, get him a copy of Mas' book.
Take care and stay safe,
Biker