He was borderline obese, probably going on the first "diet" of his life. This is possible, although it's a little faster than is healthy. It will take a while of maintenance to avoid yo-yoing.

Borderline obese is not that high a bar. At 6'3" an "obese" BMI is 240 lbs.

Just for fun, let's use that popular "3,500 calories in a pound" estimate. You'd have to run a 2,750 calorie deficit per day to lose 5.5 pounds consistently, every week for two months. That would be a 24/7 fast, not 16/8. Remember, they said there was no exercise involved either so this is purely calorie deficit. Maybe they forgot to mention a limb amputation.

My suspicion for anyone losing this much weight this fast is that a big chunk is water weight. With a big change in diet there’s a good chance that the body will quickly find a new equilibrium with the amount of fluid cells will retain, especially since both salt and glucose are necessary for osmotic pressure in cells.

I also suspect sudden weight loss from diet change can partially be attributed to mass within the colon. If you have 5 lb of dense food packed in your digestive system, and suddenly eat nothing but high fiber low calorie foods. You’re going to easily shed a couple pounds in the next day or two.

If we assume half of the weight loss was not directly calorie related, a 1400 calorie deficit could easily be 1 american meal a day. Not totally out of the realm of possibility.