Of course it's possible.Slam fire... in a modern handgun, like a SIG, firing pin block, yata yata tata... possible?
Statistically, however, unintended discharges are more than likely the result of operator negligence, rather than design defects / damaged parts.
This is why regular maintenance and inspection of a firearm is important. With regular maintenance, inspection, and function checks, it is highly likely that an individual can identify potential issues BEFORE they can lead to a possible unintended discharge.
Was there fouling / debris causing the firing pin to stick and protrude through the breech face?
Was there a problem with hammer follow that could have been identified through a normal function check?
Or, was the round simply discharged due to negligent handling techniques of the owner?
Due to the fact that the individual failed to orient the firearm in a safe direction when he sent the slide forward, I am more likely to believe that the discharge occurred due to either negligent handling practices, or maintenance, on his part, rather than on faulty equipment.