Just a simple search shows this idea goes back to 1856 and a Supreme Court case. And many more cases up to the present date, both on the federal and the state level, show the same.
So if your friends pick up the phone and call 911 and say "Hey, there's a guy over here with a gun and he's about to shoot me!" they need not be surprised if the cops wait a while, to be sure the danger has passed before they arrive. But after that, they will gladly take a report. Or at least call the coroner (medical examiner here).