Yes, your mother may have a case. There are two potential defendants: the operator of the scooter and Costco. The case against Costco would require a theory of negligence against Costco. What did Costco do wrong that caused or contributed to the accident? The better case is against the scooter driver for negligent operation of the scooter. That person's renter's or homeowner's insurance will cover them for the injury they caused to your mother.
Did your mom get the contact information for the operator of the scooter? If not, you will have to get that information from Costco. Did the store make an incident report? It is unlikely that Costco will voluntarily give you the scooter driver's contact information. But a lawyer can obtain the information from Costco.
Given the seriousness of your mother's injury, I recommend that she be represented by a lawyer. It's important on a case like this to move quickly to preserve evidence. If there were witnesses, it is important that their statements be taken while memories are still fresh. If there was video surveillance film, it needs to be preserved before it is recorded over.
Our law firm has handled many injury accidents involving motorized scooters. We have also handled many cases against Costco. You are welcome to call us for a free consultation.