I've seen a lot of people say stuff like this, but it's kinda gross. If you want to criticize things someone has said about their diet and health practices, that's fine (and hopefully, you'll cite sources for why they're wrong), but if you're going to make it about how they look, and make your own diagnosis about their health, that's not really OK.

You don't know anything about the guy's health. He may actually be sick, wholly unrelated to his diet. He could be dealing with an eating disorder (which may be related to his diet, but millions of people have fasted regularly without having an eating disorder...there are lots of ways people cover for eating disorders). And, maybe he's in great health and your beliefs about what "healthy" looks like are wrong ("put some meat on your bones" is not great health advice for most people, but it's a common refrain).

I don't really know anything about what he's said about diet or fasting. But, I know that bringing his appearance and health into it are not shedding light on the subject, they're just conflating appearance with health and well-being in ways that aren't useful.