My best friend is not eating?

It is very possible that your friend will actually go into starvation mode, where the body reacts as if there is an actual food shortage and it thinks it needs to store everything as fat. So, completely stopping eating will eventually make someone lose weight, but at a high cost. Loss of muscle m (you body can cannibalize its own muscle for energy, since protein is easier to convert than fat!), loss of energy, and loss of mental agility are a few side effects, not to mention that your friend will probably have severe nutritional deficiencies, and vitamins can only replace so much.