Maybe he has a protein and/or deficiency. Thin as a rake does not mean healthy.
Discussion
That's my point though. I don't think there's such a thing as a diet that's good for everyone and there's no way of knowing what's healthy anyway, not just by looking at someone.
I suspect when you and I eat the same salad the chemical reaction inside our bodies is more different than it is similar.
It may be good for you and bad for me. And over a population those variations are so great there's no way of designing a universally healthy diet.
I remember reading about the Vietnamese who ate 'healthy' American food during the war. It went very badly.