There's a strong argument that weight gain only happens when/if we develop insulin resistance.
First came across the idea about 25 years ago, in a Montignac diet book, and have come across it many times since. It makes perfect sense to me because insulin is the hormone that tucks excess glucose away in the form of fat. So the more insulin you have, the more fat you deposit. And what causes insulin to be released (or injected if you are an insulin dependent diabetic)? Carbs.
And the more insulin you have floating around, the more likely you are to become insulin resistant (which is where your body needs more and more insulin to have the same effect).
Insulin resistance is the main driver in T2 diabetes, and it is necessary to allow the body to lay down fat.
So I would guess that Mr CoffeeshopSugarFest is someone whose body simply doesn't develop the insulin resistance that causes the weight gain.
- doesn't mean he is healthy though! That amount of sugar has other knock on effects that can be very harmful, such as raised cholesterol, rotten teeth, an unhealthy gut biome, candida overgrowths, etc. etc. He just hasn't put on weight, so at the moment, his metabolism is coping without developing excess weight.