I can see you're confused, so I'll just continue.
- Doctors and personal trainers use people's weights to recommend lifestyle choices.
- Safety technicians and engineers use weight statistics to determine things like an elevator's proper weight tolerance vs. its available space, diving board strength, car seat mechanics, and more
- Athletic organizations group athletes into weight classes
- NASA uses people's weights when considering stress tests under extreme gravitational force
- Sociologists study macro trends in people's weight as part of comparative health reports about regions/states
It's the wrong lesson to say that in all cases, no matter how abstract, hypothetical, or anonymous, we must never talk about people's weight. But if I were the teacher/school, I would totally capitulate and be supportive just in the name of avoiding being on the wrong side of a story like this. Which, as we can see, would be wise.
At any rate, not a big deal, but I rolled my eyes at this. It's cute, but actually it was a teachable moment where more subtle engagement with the girl (what is the etiquette?) could've occurred instead of saying WOW GREAT JOB YOU'RE RIGHT. Because she kind of isn't.