What your friend did wrong is that if he really felt threatened he should have not brandished the weapon, but pulled it, aimed at the other guys, and held them in a submitted position until the police arrived.
You never just "show" a gun. You pull it, aim it, and get one of the parties to submit until law enforcement arrives.
How many times have you had to do that as a civilian?
Most criminals are not so stupid that you have to aim through a set of sights. At close quarters I wouldnt even advise it or things like a Weaver. I would and have held the gun at about Solar Plexis height, with the gun along the front of my body and my side turned toward them. IT will sting if I have to fire it, but I dont want them to grab my gun and I want my left hand free to do whatever a hand needs to do. I only had to do that once, the rest of the time simply them seeing the gun was enough.
And no way will I call the cops. These guys, not even mentioning th e DA, are rather inventive with the law at times and if they dont like what you have done you might be going to jail,no matter what, and your gun confiscated for all practical purposes once the cost of getting the item out of impound is higher than the cost to replace the item itself.
Where is the upside to calling in the cops if the crooks are gone and no harm done?