The naive thermodynamics argument doesn't say much, because: When you eat "too much" high-energy food such as your co-worker, the excess energy can be excreted

The naive thermodynamics argument doesn't say much, because: When you eat "too much" high-energy food such as your co-worker, the excess energy can be excreted rather than digested and stored. The energy that is digested may be burned off in the form of maintaining a higher core temperature (as mentioned in TFA) or making the body more active. To sum up, thermodynamics alone doesn't tell us where the "excess" energy will go -- storing it as fat is only one of several options.