“So, was it the food he ate or the working out that made the difference?”
In my opinion it is a mixture - with more on the “food he ate” - AND his age/calorific burn genetic profile.
By cutting out empty carbs, I have lost a lot of weight - by that I mean: no chips, added sugar alcohol, soft drinks, white bread. And minimizing fast food (oily fried stuff) - not eliminating.
Essentially it became that most of my meals are home made from fresh ingredients - no processed food except for bread.
It’s tough to stick to a decent diet when you aren’t close to home (working.) The vast majority of eat out places serve horrific “food.” The ones that don’t server poor quality. They have to for economic survival.