He was doing that to prove a point about the efficacy of fasting. It’s certainly not optimal. Do whatever keeps you on track though. Some fries with a refeed that give you a little joy and keep you on a fasting focused lifestyle is better than eating super low carb and quitting.
You can eat whatever you want.. However from experience the next fast is always harder if you ate carbs. Also if you eat junk, go to the gym on that refeed day.48s with junk : hard to lose weight. (slow weight loss or maintain.)72s with junk : it’s okay.