Hi. I'm Dr. Clark and happy to be able to assist you, being a grandparent myself.
I would be concerned too and believe the doctor should have done some basic tests just to reassure you (and me) that she is OK. I would insist on a urine analysis and electrolytes (blood test) just to be sure her kidneys are functioning normally as well as other systems that regulate our fluid and electrolyte balance in our blood. If these were normal then I would not be concerned and then the onus is on her caretakers to withhold her access to salt.
It would be unusual for her salt craving to be a sign of any serious illness or metabolic problem but worth checking into. In the meantime, as with kids who hoard food, eat or drink too much, etc it is simply a matter of restricting her access to salt.
I wondered about other systems-hormones, i noted from adults on the internet these were frequently an issues
I don't know if this is the place to reply?