It's pretty easy to cock at least some single actions that are holstered when getting out of a car or truck with the seatbelt. I can't see any reason to intentionally do it.
I'd probably have mentioned to the fellow that his revolver was cocked.
A guy who was the mayor in a small town where I was living some years ago had the hammer of a holstered Ruger BH .357 mag get drawn back and released getting into or out of a car. The gun went off and shot him in the leg.
