What seems to have likely happened here is when the bag was on the way out of the lost stuff storage area someone noticed it had valuables and being one of the last people in the line of steps they took the valuables and put crap nobody would miss in its place.
Such a substitution must happen early, before the bag gets inventoried, or late when it's on its way out, because validating claims of what's in the bag is a key part in the process of how lost luggage is returned. It's less the bag would have gotten back to her if the substitution happened early because "hey I'm looking for a bag full of shoes" doesn't result in any hits when the shoes have been sub'd out for garbage. Furthermore, there's decent video and paper trailer for handling of baggage because otherwise it's a smuggling/theft/crime vector and the airports sell lost luggage in bulk so they don't want the good stuff going missing. The audit trail is less good for lost baggage exiting the system to its rightful owner because it was already subject to scrutiny on entry.
It is likely that this does ultimately get tracked down because lost luggage, at least in the US, is handled mostly by the airports themselves so the airline and airports will want to point the finger at each other and the truth should eventually come out. I'm sure when all the dust settles they'll fire a line employee and offer her $500 worth of air miles or some other completely unsatisfactory outome like that.