Of course some blame needs to be on the person who brought a loaded firearm to the employee. If it's your carry weapon it doesn't come out of the holster. If you are bringing it in to be serviced, you clear it at the entrance. Nobody should be handling a loaded firearm in a gun store, unless responding to an attempted robbery. Most stores have clear stated and signed policies about when and how weapons will be handled.
Why? Because if it's to be serviced it will be handled, and that may involve working the action. I can see exactly how this happened, the employee went to work the slide (perhaps to clear the weapon just to be sure), but it was still and perhaps the employee's hands had gun oil on them, the slide slips, the slide slams back onto the already chambered round, and being a .45 (1911 most likely) the hammer was partially pulled back by the slide being partially pulled back, it slams home and you have a negligent discharge.
Now this article doesn't state what the expected status of the weapon was, the employee could be entirely at fault by working the action with a loaded mag in it, and may have even placed the booger picker on the trigger. But I can also easily see how this was more the fault of someone handing an uncleared weapon to the employee. More details are needed before blame can be assigned.