In order to get a real keyhole, didn't he either have to have either (a) some form of failure in the normal spinning motion of the bullet, or (b) some damage to the projectile itself?

I don't get how to get (a) unless there is damage to the barrel / rifling

but (b) might be possible if he also crimped in some odd way and then with the overpressure had destruction of the bullet as it freed itself from the case in a violent fashion --

Personally, I have accidentally overloaded three rounds in my life (too heavy a bullet) on a .223 and that was probably only by a grain or so, and I would never even think of doing that stupid a move again. I was a newbie and didn't recognize the primer damage immmediately when the little curl of smoke came out of the action, but after the third one, I caught on and pulled all those rounds apart. And if I remember right, that was actually INSIDE the pubished data, but I had the bullet seated out closer to the lands than the published, so I got higher pressures..... there are a zillion ways to foul this up and you are right to politely but firmly suggest he take responsibility for grossly and intententionallky overloading the cartridge.

Now it would be wise for him to carefully inspect the bore, throat, and bolt. If no signs of damage, he might try some factory loads!