TWO DOGS, FIRST DIED, SECOND SURVIVED

The owner brought two dogs to the veterinary clinic. Older 14 year-old dog suddenly died there and the other 6 month-old lived, but has apathy, bloody diarrhea and vomiting. The owner thinks, that someone has poisoned them.

ANAMNESTICS:

Vet: Did the survived dog eat something? 
Owner: "He did not eat and drink much, just a little bit and after that he vomited."
Vet: What did you give him to eat?
Owner: "Just regular food like the bones, granules. But my dogs mainly cought the rats afterwards"
Vet: Did they defaecate?
Owner: "Yes, the faeces was with blood."
Vet: When did the blood faeces start or how did the dog look like before the change and did you notice anything suspicious?
Owner: Both dogs had the same symptoms. They were weakened and also had the diarrhea, later with the blood, and the vomitus. The vomitus was like a foam and it smelled awful "

The survived dog is 6 month-old  and he is the patient of this veterinary clinic from 8 week-old. Each vaccinations and papers are alright.

The died dog was 14 year-old and he was also from 8 week-old the patient of the veterinary clinic. Each vaccinations and papers were alright.

What will the veterinarian do?