We all have etiquette-proof people in our lives, hence there are recurring characters in the stories we tell here. However, there are degrees of this: Most of the etiquette-proof are not as egregious as the Dresser Queen, Office Annie, Nichol, or K'nnihave. This question is what we do about the others.
When I posted the poll about my cousin's son in etiquette school, I told some friends in my face to face life as well. Most of them were impressed, some realized they had no idea that such schools existed, and one person was shocked and asked "Why?"
I knew by the time that question came up that my cousin wants her son to be able to feel comfortable interacting with everyone he is likely to come into contact with in future, including people from more affluent circumstances. The explanation was met with complete silence, so I changed the subject.
The person in that conversation is the younger sister of an old friend. She's a good person with a generous heart and a good sense of humor, but seriously lacking in table manners. When putting gravy on her food and seeing it was about to overflow, she licked that area of the plate. She does nothing to stifle the sound of belching after eating, excusing it with "[ethnic] compliment" (of an ethnicity that none of us have). I worry about how this will rub off on her sister's son who will be 12 this spring and is already a challenge as he is a picky eater who won't try even a forkful of anything unfamiliar.
I don't have a dog in this fight but if I were the mother of that boy, what would you recommend doing?