I would suggest the facility was negligent in their security arrangements, as far as wrongful death (again, it’s a pretty dubious “if”, that it goes that far), it would be like somebody dying because the building wasn’t up to code when an arson came by.
My assumption is, though, that there’s a budget-rate warm body security guard; and between shit pay, shit training, shittier oversight… the guard couldn’t be arsed to care. (Alternatively, the guard was going to sell them for drug money.)
The good news for the facility… if their lawyers were any good in that contract they’d have gotten an indemnity clause and can pass that buck. (Liability is a bitch; and she hits hard. The security company will probably go poof unless they’re the size of G4S or Securitas)
In any case… personally, it doesn’t rise to wrongful death, but I can see a need for nuance. I would, personally, suggested the couple treat it as property, similar to a safety deposit box.