I'll slow this down for you one more time.
The trigger pocket was over-molded and forced in so far that when the firearm is inserted into the holster, the kydex indentation actually catches on the trigger, effectively pulling the trigger. The officer on scene eventually demonstrated this to this person.
That's directly from the OP.
If the holster deformity was so grossly evident and obvious and the malfunction was so easily repeatable that it was "discovered" while still on scene, then the user probably lied about test fitting the weapon in the holster because he's embarrassed as shit to have shot himself and KNOWS he should have inspected it properly.
You even assumed, based on the evidence given to you, that no test fit occurred. So that right there tells us that even you were wondering how somebody could miss such an obvious deformity that it would allow the trigger to be depressed while holstering.
Now you're sitting here, essentially scrambling for ways to exonerate your friend. Now he did test fit it, maybe it was a positional thing he couldn't see, maybe the retention click masked the sound of the trigger, maybe it's only intermittent. Come on man... I'll buy you a box of straws so you can stop grasping at them.
You know as well as any of us do that when a trigger is depressed to the point of firing both the sound of it and the feel of it in your hand are completely different than the sound and feel of the click that's produced by the retention system of a holster. Not only that, but let's consider the additional resistance incurred by the weight of the trigger pull as he was stuffing it in the holster. If he was just holstering the gun, that additional 5 or 6 pounds of resistance pushing back should have been a major red flag.
Maybe I'm totally jacked up even more so than originally thought, because I pay attention to things like that. I holster and unholster at virtually eye level while I inspect the fit and find contact points that might cause wear down the road. I put it on and do holster, draw, reholster drills with a dry weapon. I walk around the house for a few days with an empty weapon holstered so I can get the fit and feel just right. Yes, I randomly do quick draw drills during those few days. I'm fully aware I look like an idiot while doing so. However, I now have 100% faith that my new holster will be reliable when I need it to be because I'm weird and an idiot.
I have an extremely high level of attention to detail. You know this. I am obsessed with making sure everything is going to be done properly. It's why I work on my own vehicles, guns, residences, appliances, furniture, electronics... you get the point.
So if someone fails to do even the most basic of assessments on a piece of gear, and it fails, then to me they are negligent.
EDIT: Just thought of this other thing I do that I'm willing to bet your friend didn't. Well... technically he did... but he didn't understand the assignment...
I holster the weapon while it's cocked to see if the trigger gets depressed.
One more test that could have saved him from all of this.