I agree that's really impressive if he's going to the gym 2-3hrs/day.
There are multiple reasons I agree with the above poster that this isn't going to happen.
First is I think it's literally not possible without repeating all his USMLE testing. And I'm not sure you can take them this far out of med school. Even if you could, there'd be a lot of time/prep into studying for those. I can't imagine how much time I'd need to study to re-take Step 1 right now... And then there's still Step 2 and 3. And After he passes all of them he could then apply for residency (he can apply without Step 3, but I can't see any program remotely considering him without having all three). So that's a few years already between studying/taking tests/application cycle. Add on another 3 years for the residency, as he won't get credit for rotations from 26 years ago.
So probably looking at 5 years +/- until board-eligible. At that point he'd be 65 if he's only 60. How long does he plan to practice? The trade-off doesn't seem worth it to me.
I also don't see any residency program taking on a resident in their 60's. Call it ageist, but they are unlikely to think someone in their 60's can keep up with the younger interns, physically or mentally. After 26 years in practice, your friend has likely also reinforced a lot of his clinical thinking/practices, so it's going to be significantly harder to train him compared to a fresh med school grad.
Being terminated from residency is another red flag. Not insurmountable, but in combination with all of the above, I really do think the odds of success are near 0. I however, am not an adcom so he doesn't need to take my word for it.
Why not apply for jobs with the VA, DOD, State Hospital system, IHS, etc. I believe quite a few of those only require a state license and not specific board certification (hopefully someone can correct me if I'm wrong), and they would come with benefits/job stability.