Carla Cheney told the Canadian Broadcasting Corp. she saw man leave his parked truck with a dog inside to shop on a hot day at the Kemptville Walmart where she worked.
"I said, 'Is this really happening? I'm going to give him about five or 10 minutes and then I'm going to call the police,'" Cheney said.
She called police, who came and took down the man's license plate and went inside the store to find the driver, the CBC reported.
As the man was leaving, he allegedly pulled his truck over to where Cheney was sitting at a table with colleagues. Cheney said she told him, "You shouldn't leave your dog in the car."
He told her it was none of her business and said he wouldn't shop at that Walmart again.
Cheney said she was later called into her manager's office where she was told she was terminated. The reason the manager gave for firing her was not reported.
"Now I'm worried that other associates won't say anything and they'll be afraid that they'll lose their job if they do say something," Cheney said.