I don't know, and feel uncomfortable even replying to you because....I don't know. But I do know that you can encourage the production of human growth hormone by doing high-intensity exercise, something that sounds really, really, stupid to suggest to someone with your friend's circumstances. I'm just winging it here, but maybe, if he can manage it, even twice a week for 10-15 minutes, his HGH might rise and help to stimulate new blood vessels in his extremities....mebbe? Exercise of any kind does that, but not all exercise, like walking, stimulate a lot of HGH. They just encourage more vascularization to provide the muscles a chance to grow to adapt to the increased burden you're imposing on them if you walk more often and longer.

I hope you get some solid answers, but also that you come back and tell us what you have learned so that we can pass it on.