How to correct someone by not being rude. Try by being close to them, not too close but be there to make them awake that you’re watch their every move, humans feel bad when they do something they know is bad.
One example... let’s say your friend is messy in the kitchen, or leaves the smoke running, just sit by the kitchen, do nothing but just be there, they’ll be so clean and very polite, they’ll clean up after themselves. Believe me. It just happened.