It's the old prejudices coming out...
fat/overweight = unhealthy, lazy, stupid etc
slim/'normal' weight = healthy, active etc
Neither of these beliefs is necessarily true and to accept them as fact is dangerous, not to mention offensive. It is a fact that maintaining a stable, larger weight is healthier than yo-yoing up and down all the time. It is also true that some slim people maintain their 'healthy' weight/appearance by laxatives, vomiting, etc.
I hope that these myths are knocked on the head and this woman can be left to get on with her job.