My friends brother euthanized his dog himself. What should I do?

My friend lives with his older brother. His brother had a dog who kept have seizures for a long time. My friend said that he woke up yesterday morning and the dog was laying on the floor, and wouldn't respond to him. His brother tried to make the dog walk, but he couldn't walk. My friend said that his brother took the dog out back, and shot him in the head. I'm pretty sure that's illegal, but I have no idea where they live so I don't know how I would report it.

My friends brother euthanized his dog himself. What should I do?
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