Aside from the obvious mistake in lowering his guard to the "customer" the officer serving as backup also made the mistake in retreating. As most of you know I am a big proponent of retreating while drawing and firing as I move to cover. In this case and most cases that are at contact distances, with the bad guy already pointing the gun at you, it is suicidal to retreat. When faced with this situation your odds of survival are better if you charge and get control of the weapon. A friend of mine ran sims drills and those who retreated had a 100 percent get hit ratio. Those who charged suffered only 60 percents with many of the hits in non life threatening areas.
If you fear this kind of situation get yourself some hand to hand, disarm, and grappling training. Here at my work we practice (far to rarely sadly) what we call going "psycho". It is nothing more than charging your attacker and fighting for the gun. The psycho name was born because A. charging a man with a gun pointed at you is crazy. B. Once we get on the bad guy we bite, head butt, gouge eyes, pinch testicles, scream, and just about anything else you can think of to win. Everything in our practice is run very slowly with only simulated contact so as not to injure your partner. It doesn't give you 100 percent reality but it does teach you to take what the bad guy gives while focusing on keeping his gun under control.