I would also be concerned for my safety. You never know others extent of training or experience, so I always consider fellow shooters as "new" (until they show
me otherwise). I would have said something to them politely, probably not pointing out that there has been many accidents with their make or model firearm, but that its just common sense, and maybe range policy, to have the slide locked back when your not on the line. If they continue to show ignorance in future encounters, be as rude as you need to.
If you ever think you have it bad, the outdoor range that I am a member of, has a BAR
in the clubhouse...
Although they do have "dont drink and shoot" signs everywhere