Since he didn't eat much carbs or protein he probably also lost a bunch of muscle mass. His metabolism likely suffered too. Sustainability is everything. If one wants to lose fat and keep it off, it has to come from a lifestyle change, not a quick diet fix. A bunch of idols do this too. They barely eat anything and do a ton of cardio. Good luck trying to sustain that because you'll need to eat less and less and do more and more cardio to keep losing weight, and a lot of it will be muscle mass, not fat. In the end, your body composition will be the same, if not worse. Having balanced meals (ratio should be about 1/2 vegetables/fruit, 1/4 protein and 1/4 carbs for the majority of meals) and mainly doing resistance training with some cardio mixed in is a much better solution.

Of course you'll feel like you're starving to death when you're an adult eating less food than even a child would eat. You can eat more than just 6 rice balls per day and still maintain that healthy body. There are many healthy diet options out there.