How can u be charged with a hit and run if the driver of the car stopped and called 911 and was on the scene until the cops arrived?

A friend of mine was driving home on a dark road and a drunk guy with all black on was walking in the middle of the street and my friend happen to hit him.he called 911 waited for the copsto arrive. He was not charged and was not arrested. After thepolice did a sobriety test.They impounded the vehicle did and investigation. We thought that was it.case closed .a freak accident.Then months later,my friend has a warrant out for his arrest for a hit and run...how could that be?