Ok, please tell her to follow the Vets instructions. Give the meds, make sure the pup is eating and drinking water. Get a syringe and force children's plain pedialite. Get nutrical.
Honestly, this pup should be with her (not the boyfriend) or even with you, since you have Yorkie experience.
She can go after the Petstore for the Vet bills, as there is an incubation period for this and I will bet anything that the pup and other pups at the store have it to
Please, whatever she does, please do not return this pup. The store will "return" the "defective merchandise" to the puppymill and they will destroy it - sorry to say, but that is the truth....
...also, some Vets will do a payment plan, she needs to discuss this situation with the Vet, tell the Vet where the pup came from, etc....