Don't save proper etiquette for strangers. Friendships need to be nurtured with good manners and respect. Here are some guidelines and tips.
The Etiquette of Friendship. What social norms have you found yourself obeying/ violating/noticing as you go through adult life? Which ones.
Perhaps someone that was a good friend for a long period of time has decided to move on. There should be no judgment in taking another.
Remember the basic success principle underlying all manners and If your children are invited to a friend's house to play, they (and you) My children know better—so why do I see adults exhibiting such poor behavior?.