A friend of mine is trying to lose weight. He's taking weight loss pills and drinking nutrient shakes, however he isn't eating for days at a time.

No magic Solution. Weight loss is just plainly difficulty. Genes play a huge role in determining our weight. Unfortunately our sedentary lifestyle does not help. Starving oneself to lose weight is not a great idea though. Eat a balanced diet but remember portion control is key. Exercises help a lot too. Just minimized fructose and refined sugars.
Family practice . As long as he's getting nutrients and vitamins with the protein shakes he should be fine. Make sure he is drinking alot of water. It's always wise to see a dr first before dieting.