I have never had a stroke, but I have had a serious heart condition that lead to hospitalization. When I got out of the hospital, I had some issues. Mobility was a huge one - I will never forget the first day of 'rehab' when I decided to walk about two kilometers home from a coffee shop. About halfway, I heard these two older women coming up behind me. One used a walker. I tried my absolute best to keep up with them but I couldn't. They passed me like I was standing still and the effort of trying to beat the woman on a walker was so extreme that I had to sit down. Hell, I almost called my Mom at work to get her to come and drive me the rest of the way home.
There I was. 40 years old. Lying on the fucking grass on the side of the road. Genuinely asking why I had to survive if this is what life was going to be like.
I had to rehab two things. I had to rehab my body, but first I had to rehab my brain. I have never and will never forget the feeling of 'used to' while I was rehabbing my body. I 'used to' be able to run. I 'used to' be able to walk faster than little old ladies on walkers. I 'used to' be able to eat whatever I wanted. I 'used to' be able to mow my fucking lawn. I 'used to' be able to walk up a flight of stairs without taking three breaks on the way. I was paralyzed by 'used to'.
You're his friend so honestly, the best advice I can give you is to encourage the hell out of him. It sounds like he has come a long ways. I have no idea what rehabbing a stroke would be like, but I assume that it's significantly worse than rehabbing a heart. If it is actually worse than rehabbing a stroke, your friend is fucking hardcore. Seriously...this man has come an incredibly long ways and deserves some serious praise.
I'm a sample of one, but that kind of praise really helped me. Every single thing about rehab sucks. Praise makes it suck less.
Edit - I'm much better now. Hell, a few weeks ago, I told my psychiatrist that I was in serious danger of qualifying to run the Boston Marathon by the time I'm 45.