Story highlights Kelly O'Connell adopted Charlie as a puppy

He was her companion for 15 years

(CNN) Charlie was Kelly O'Connell's "best fur friend."

"We had tons of adventures together," she told CNN.

So when she learned her canine companion of 15 years was dying of a brain tumor, she was devastated.

"I adopted him as a puppy," she said, "I was 19 years old."

As much as she wanted him to be with her forever, she didn't want to extend his life in suffering.

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