Earlier quoted context omitted.
So your friend was unable to digest at least a thousand calories a day, likely more, and didn’t see fit to mention the endless diarrhea they must have been experiencing? If you’re spewing a thousand undigested calories out of your ass every day, you should know something isn’t right. Also this is still calories in, calories out .
Yes, nobody is disagreeing with physics. But a discussion about weight gain/loss that always has people saying "calories in = calories out" is like a discussion of every business saying "well profit = revenue - expenses". Obvious but not helpful. The interesting thing is discussing how to modify those variables.
Address the core issue first before arguing about minutiae. The emphasis on relatively low impact factors is especially unhelpful and unhealthy because people latch onto all these discussions as justification for why they can’t lose weight. Sure, an obese person might have a low metabolism[1], but if they eat 3500 daily calories and live an entirely sedentary lifestyle, those are probably the major causes.
[1] But they don’t. Study after study have shown that obese individuals have high metabolism because metabolism is tightly correlated with total body mass.