Kennel cough is contagious to other dogs. I expect that, if your parents are getting a puppy from a breeder, the breeder would not want the puppy to be exposed to active kennel cough. If the sheltie just started antibiotics, I would think she would still be contagious. I would suggest that your folks call the seller and explain the situation and decide on an alternative pick-up date. Be careful not to expose your pup either!