He will most likely be micro counting calories everything he eats and also not working a full time job like the rest of us.
Genetics aside if I had 24/7 to spend on just training eating sleeping and sponsored by a nutrition company ill look pretty good too. Still its not much of a life to live this way imo.
I would do the diet around summer 8 weeks and chill during winter carrying more bf% and less calorie counting.
Health wise - low carbs diets or calorie restricting leads to health issues such as cardiovascular disease due to lack of fibre. Constipation.
personally I have kidney stones from years of dieting with keto diet imagine these low % bf guys