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Hi there. Thanks for the question. Let me see if I can help. It is impossible to tell for sure without postmortem exam...but it sounds like the puppy might have had parvovirus. This is a virus that causes severe destruction of the intestinal walls which leads to fever, inappetance, and bloody diarrhea. Even if the puppy had started the vaccine schedule for dogs but was not through with it, it is susceptible. Pit bulls, Rottweilers, and German shepherd breeds or mixes seem especially susceptible it it can happen to any breed. I hope this helps. I'm so sorry for your friend's loss.
So small at 6 mo old. There could have been a congenital abnormality with the puppy. Or it ate something it shouldn't have and had an intestinal blockage...or intestinal parasites could have caused a gut rupture. These are all other possibilities besides parvo. Again...depending on your vet, you may be able to get a postmortem exam done to tell you what happened.