For another anecdote, I have been drinking black coffee, and only black coffee, for the first four to six hours of the day. Sometimes water as well.

I have done this for about six years without damaging my digestion in any detectable way. I would bet money your friend has or had a Helicobacter pylori infection; treatment is antibiotics.

I tried the same for for about 6 months but had to stop because of stomach ache. I'm not infected with helicobacter pylori.

Sure, people differ greatly in acid sensitivity and other factors that make no one diet right for everyone.

The stomach ache went away when you quit? If not... are you sure you don't have helicobacter pylori?

Yes it went away and yes I was tested multiple times for helicobacter pylori.

That definitely sounds like some predisposition. The whole idea around intermittent fasting revolves around eating in a specific time window, outside of that you don't consume any calories at all. Many people I know of do this, in order to lose some weight or gain muscles without adding too much fat, never heard of this problem.

Coffee can be quite acidic which with people with overactive stomachs can cause issues such as acid reflex, ulcer etc.

Instant coffee tends not to be as acidic same goes for cold brews and there are other ways to neutralize the acidity.

If you produce too much acid then intermediate fasting is not a good idea to begin with unless you also take supplements to control your stomach acidity.

FWIW I used to get heartburn pretty regularly and that has completely gone away since I switched to a plant-based diet.

Not everyone can take coffee (or tea for that matter) on an empty stomach. And of course there are many levels of "strong".

The other large confound is he was in the military service during this time. And stress can totally mess up the GI tract.