people are forgetting he won't be shifting from a 500 calorie diet back up to normal. He's eating 500 calories of external food, but his body will be eating a LOT of calories, literally eating the body fat. call it 2 stone in a month, 28lbs = 28x3500 calories(1lbs fat =3500 calories roughly speaking). thats 9800 calories, or infact, just call the month 28 days for simplicity and you're talking about your body eating 3500 calories of fat a day.
In reality its not quite that high, but you'll be surprised. Just like when you eat more food, you'll put on weight but your body WILL adjust its metabolism to compensate. So this diet is essentially shifting the body to getting its main food sourse from body fat, and once its in body fat mode it just keeps on eating an endless supply of body fat.
The danger comes in changing food source and using a different process to get energy. IE high carb diet will be incredibly dangerous, building up calories mostly from high quality fats in the diet would be pretty safe.
Obviously theres other things to take into account in the math, some/quite a lot will be muscle being lost, lots will be water weight aswell. Infact carbs take in so much water weight with them, that the figure could be low as 1/2 of that, but even so thats still 1500 or more calories the body is still adding to the 500 calorie food pack.