My brother seems to get sick whenever he takes time off from work. Like your friend, he is a small business owner, and often feels that the business can't function without him for a day sometimes.
Maybe with folks like your friend and my bro, where work is a 24/7 concern in your head even if you're not physically present there, the body is cranked up on the adrenaline of "I have to get this done right now, and only I can do it right" when AT work, then the body just kind of collapses when there's a moment of letting down that guard....
I feel for your friend and his family. It's rotten to get some time off and then spend it feeling bad. I think you put it well -- as soon as he slows down his body has time to let whatever's been brewing to "come to the fore."
My brother's gotten better, though, since getting married again and seems to be able to leave work worries behind more than when he was divorcing and then single!