He should not fire a "warning" shot. "Warning" shots endanger innocent bystanders and expose you to legal repercussions for no particular purpose.
Either let the perp run if that's what he is going to do, or shoot him while he is still facing you and say "I feared for my life" and nothing more.
Carelessly-fired warning shots endanger innocent people, true. On the other hand, if the person would have been legally entitled to e.g. shoot a plastic soda bottle which was sitting at the spot on the ground where his bullet hit (which would have been permissible in many rural areas, but forbidden in many urban areas), the fact that the sight of him shooting that area of the ground might disturb a burglar should not change the legality of his doing so. Suppose, for whatever reason, the guy had a bullet-catching device on his porch (similar to the ones some gun repair shops use for functional testing when accuracy is not a concern) and had fired into that, such that he was basically using the gun as a noisemaker. Should that have been illegal?