I realize I’m a bit late to the party here, but I’ve had very similar experiences, and I’m not close to 60 yet. I would chew and chew, and couldn’t get myself to swallow, except for straight sugars (read: M&M’s, Skittles, that sort of thing).
This is NOT a medical answer as I’m not a doctor, but I think it’s overexertion. In essence, he’s overdoing it, and needs to do things like slowing down his hiking pace, going less miles, staying hydrated but NOT over-saturating himself with water. Also being in shape.
For me, I was 20 lbs overweight and trying to tackle routes I could handle 20 years ago when I was younger and in shape. I’d tackle too many miles and too much elevation gain (no problem with altitude, but it can sure take a lot of energy). Then I’d have a hard time swallowing/eating, and my body didn’t want to ingest it. But I’d drink water—too much—and then I’d want to throw up. So I’m not really sure if dehydration is even part of it; for me, it was squarely overexertion and being out of shape.
For your friend who is 64, age may be catching up a little bit (I say that with no offense intended). He may just need to throttle back. Less days on the trail, less miles, less exertion might just do the trick.
Oh, and I’d get down off the mountain and could down a 20-piece chicken nugget box and large fries without a problem or hesitation. So it wasn’t a problem with my throat, reflux, etc. I was just pushing myself too hard, and my body said no bueno.