I would get another opinion... I really wouldn't trust a Petsmart vet for breeding/litter advice. I would trust Mary waaaaaaaaaaaaaay before a Petsmart vet. She knows what she is doing.
A c-section is a real possibility and she would have to be put under anyway, plus all the extra risks that go along with the pregnancy... If it were my pup, I would be seeking another opinion and getting her spayed now.