Far as I can tell, someone wrote:
>no feet on the table when the food arrives,

Yikes! Up until the food arrives it's okay?

When I was a kid at camp we would sing a tune whenever one of the councilors leaned
his/her elbows on the table.

"Get your elbows off the table Uncle Bill (or whomever)
(Repeat.)
We have seen you do it twice and it isn't very nice.
Get your elbows off the table Uncle Bill.
Stand up, stand up, we won't shut up 'til you stand up.
(repeat until the red-faced offending adult stands, then applaud.

Having 120 eight year olds teach you manners was humiliating, to say the least.

Seriously, perhaps a strongly worded anonymous letter will do the trick.

Dear Joe,
You're a nice guy, and lots of people like you, buy you have a VERY annoying habit.
You slouch when eating. You lean on your elbows and it bothers everyone.
Grow up and knock it off.
signed,

Your friend,
Well, lots of your friends who don't want to embarrass you.