When faced with a situation like this, rifle vs. handgun at a longer distance, should you make the choice to intervene the only viable tactical option is to close the distance to your effective range.
Now that may be 75, 50, 25 yards or whatever but you bring yourself and your weapons platform into your comfort zone so to speak. However, once you commit you are commited the minute the shooter notices you be prepared to engage at whatever distance you are at.
Gabe and his crew routinely train for longer shots with a handgun and are great supporters of micro red dot sighted pistols which are easily capable of making this type shot. Depending on your skill level and type of firearm 100 yard hits on a man size target are by no means impossible. The gentleman at the scene did what he thought best at the scene, at that time no one can fault him for that.