You teach your child that different people have different lifestyles and that in your household, you all do things a certain way. This does not make you right or Kelly wrong, but you like your home orderly so that is how you expect him to keep it.
When kids venture out of their homes, they are introduced to many different things and one of them is to see how other people conduct their lives.
My son went to a sleepover birthday party. When I picked him up the next afternoon, the mother of the birthday boy said that my son was the only child out of 8 children who brought his plates up to the sink, thanked her for the food and folded his blankets in the morning. That is because he was taught to be responsible for himself. Just keep teaching your child to clean up after himself and when he does, phrase him and let him know you are proud of him.
Let him know that different people have different rules and he is to follow your rules at your house and clean up after himself at Kelly's house.