Give them the confirmation number. Have them call up DISNEYLAND. Perhaps talk directly to the Grand Californian. And tell them the confirmation number. And have them request some particular thing they would desire for the room. Such as near an elevator.
By talking with the people at Disneyland reservations or the people at Grand Californian they are not talking with
DVC. However they should be listed On the reservation, and should be able to access that through Disneyland with their confirmation number. This will prove to them that they have an actual reservation.