Ugh, I HATE petshops that SELL dogs! I'm so glad your co-worker was able to save this poor puppy.
It's pretty typical for petshop puppy mill dogs to have diarrhea. My vet was (unfortunately) the vet of choice for our local puppy mill seller (Pet Stop) and we treated so many sick dogs for them. A majority of them had kennel cough, coccidia, and/or giardia...or a combo of all 3...because of the transportion/conditions of so many puppies crammed in a truck on their way to the store.
Sadly, we had a few that had parvo and died in our hospital.

I HATE puppy mills and stores that sell these unfortunate pups.