Oh, I think the male GSD that got her meant for it to happen.
But whatever, the dog is going to deliver in 2.5 weeks. She needs to have strength to push the puppy out. The puppy needs nourishment to grow properly so that it does not have problems down the road due to lack of nutrition. I think that is more of a worry than the delivery. Everything you try to do to keep the puppy smaller might back fire and you still may need a c-section.
I delivered an only pup. The x-ray said on very big puppy. But the pup was only 20 ounces, good sized, sure, but not out of the ball park. She had previously delivered puppies larger than that in a litter. And I let the bitch eat all she wanted. Remember that a lot is fluid and the placenta. The pup may only be 1-2 pounds, but they gain more than that. I did have to rush her to the vet and have them help get the puppy out, but at least a c-section was not necessary. She normally weighs 68-70 and a week before giving birth she weighed 76.