When you don't eat does your body burn fat first or muscle?

My friend is obsessed with losing weight. I think that she might be anorexic. She eats 800 calories a day. She says she enters starvation mode. She says that her body muscle burns her muscle instead of her fat. I tell her that your body has to burn the fat first to start burning the muscle. She doesnt believe me. So can I get some professional information or aid. This is directed to nutritionist, dieticians, etc, but anymore could be helpful!

Your body never burns one primary fuel source. It burns carbs, protein, and fats at the same time.

Your body would be burning muscle and fat, you'd lose mostly fat but a good chunk of muscle as well.

Your body starts eating your muscles before it starts eating your fat because it goes into a "starvation mode" if you will and your body stores your fat. thats why you see bodybuilders eating every 2.5-3 hours. Your body starts attaching your muscles after 3 hours of not eating.

Well your body only burns muscle as a last resort because you need muscle to move & walk. Fat is there for hunger emergencies so of course it will burn first. you need a daily intake of 2000 calories to stay healthy.