As a (retired) pilot, I have to say I feel the medics are right in treating this seriously. His 'health diet' doesn't sound too radical.
Having said this, even when I was fit, (judo 3 nights a week.) trying to 'crash' away those annoying few pounds would sometimes leave me greying out if I stood from crouching suddenly. It almost always was the case, and a small amount of sugar fixed the problem on a temporary basis. Normal diet, and it never happened.
Some 30 years ago there was a flurry of concern about lorry drivers in the UK fainting at the wheel. They were advised to carry sweets/candy just in case they felt feint.
An odd kind of logic.
I think anyone up the pointy end should be able to steel themselves to have any kind of procedure. But, now I come to think of it, I have a pal - 40years on Boeings - that can not look at a needle going into his arm. Difficult.
If I was in that position, I'd go for full fitness training...be able to cope with anything they'd throw at me. If I couldn't manage that at that age, I'd have to concede their concerns were valid.