How old is your foster, and what kind of health?
Older dogs, particularly older vaccinated dogs, should not experience the same severity of illness as a young dog. In fact, if the dog is vaccinated, I would expect to see just what you saw, some slight symptoms and then the body responded by upping antibodies. I would expect that if you titered your older dog to check parvo antibodies, they would be pretty high. The exceptions to this in older dogs is if the dog has a problem with their immune system, or if the strain of parvo is particularly virulent or not covered by the dog's vaccination.
Once a dog has had parvo, they do not need to be vaccinated for it, and they cannot catch that strain again, it is a viral disease.
I really hope that your foster makes it, so devastating, fingers crossed!