I would take her back to the breeder/vet tomorrow and give the lady a chance to make things right. It wouldn't bode well for her as a vet or a breeder not to do right by you. You have a contract, and obviously any other vet would tell you to take the puppy back...
While I'm not sure why she would send any puppy home when sick, perhaps Kali hadn't had the runs yet at her house. Regardless the puppy now has meds starting her on the way to wellness. Her msg sounded like she was willing to work it out. Just my 5 cents worth...
Like I said in another post, I remember one of the puppies having diarrhea on a day I visited but I did not pay attention to it then and she told me "they have diarrhrea from this bacteria in my backyard", i also have text messages from her saying it was from it.
Like you said thankfully she now has a treatment to get better but some people are not understanding my point here: It is not about the money. I let it go and told her i keep her posted on any improvements, i honestly don't want to make this bigger than it is but it still bothers me a bit.