I would be cross if my DC refused to eat at someone`s house, whatever it is.
I always tell them they have to at least try it, eat as much as they can, and if it is really impossible for them to eat more to say: "Thank you, it was delicious but I am not very hungry today".
My oldest DC is a fussy eater, and I do not encourage pandering to it, so if he goes to someone`s house I just tell the mum to give him a small portion, so as not to waste any food.
Also, I am SICK of smug posters who think fussy children are the result of pandering parents.
I have one fusspot, and one child who eats anything, they were BORN that way. The fussy one refused powdered milk as a baby FFS; and any food from a jar/tin. I do not encourage the fuss, or pander, but he does not eat mince. What can you do?!
Any visiting children at my house get whatever I have in the fridge, if they do not like it they can have bread and buter.