Well there is admittedly some complexity and nuance, as well as situations where a gun can fire without the trigger being pulled. But generally the only way it was going to happen with a revolver was if he dropped it and it landed with enough force for the hammer to slip over or break the sear and drop on it's own.
There is no excuse for a properly maintained weapon to malfunction in that way, though, and there is no excuse for the weapon not to be properly maintained.
(I might also add that revolvers can have accidental discharges if, say, some clueless monkey has decided to lighten the trigger enough that when cocked if can fire if wobbled enough, or if it's old and poorly maintained and slips off the cocked or half cocked notch (or their thumb slips when cocking it) and doesn't have a hammer block. No excuse for either of those conditions).
In WW2, someone had an accidental discharge with a revolver when it fell something like 30 feet onto the deck of a Liberty ship, and so they modified the design of service revolvers so even that wasn't enough anymore. 70 years ago.