There are many conditions that would cause an otherwise healthy person to gain weight, and not be able to shift it. Sometimes it's due to medications they're taking, sometimes it's age-related and other times it may be hormonal or adrenal problems. It does not mean that person is ignorant or uninformed about healthy nutritional guidelines.

A person's appearance should not be a factor when you are receiving medical advice. That's a pretty limited/ limiting way to perceive people, and the help they can give you, basing it all their appearance alone.