I can't see how the rim hardness would contribute to a "slam fire". The original condition of slam fire in an AR was seen in the late 1950's with the old, original heavy firing pin - and even then it didn't rear it's ugly head until soft commercial primers found their way into the production steam.
I ran a season of SR with BR primers and not a single slam fire or doubling incident. I switched back to CCI 400 since there was no appreciable performance increase.
It is technically possible to encounter a slam fire with poorly seated (high) primers, but unlikely due to the construction of the anvil and cup. A rifle length spring produces about 15 kg max of force to close on a maximum case. Compare that with the force to seat primers. The whole bolt face would strike the whole surface of the primer and push directly forward. The height of the primer over the base would have to be higher than the headspace delta and it would have to be seated hard in the case to get any deformation at all.
I would rule out a trigger mechanism malfunction before looking for the slam fire boogie man. It is far more likely that the hammer followed the carrier or was released on closing due to worn parts or a maladjusted match trigger.
If the case was too soft you would see marks similar to high pressure signs (ejector and extractor cut extrusion), bulging at the last point of chamber support and possibly deformation from the extractor strike on closing or extraction. Worst case is soft brass would cause a rupture or breach explosion. I have seen soft or improperly annealed cases from nearly every major manufacturer.
He may want to check that firing pin protrusion is not excessive - but again that alone would be unlikely to cause a true slam fire.
