"What you tolerate becomes your standard."

I learned it the hard way - working at schools. Just show kids once you're OK with someone acting up and voila - the whole class is out of control.

It sucks when this happens with friends or "friends". You see how things are slipping and I don't know if there is a way to get back to the "higher" standards you once had. It gets into a cycle of treatment you are not happy about becoming a norm and you playing along like that's how it should be. At that point bringing that friend to have a serious talk is not even an option...perhaps because you know what will happen.