She walked up to the man, still in her own clothes, and told him he shouldn't leave his dog locked up in the car with only an inch of open window to breath through.
"The man said it was none of my business," she recalled.
Carla, a pharmacy technician, figured that perhaps it would be the police's business, and decided to give them a call.
An officer arrived at the scene and wrote down the dog owner's license plate number before entering the store to speak with him.
The following day, part way through her shift, Carla was called into the manager's office and asked to explain the incident with the dog.
This was the second time in as many weeks that Carla had spoken with the manager about people leaving dogs in hot cars, and the last time didn't go so well.
"I was pretty upset and I said to my manager, ‘What do I do?’ He said it was none of our business and went into the store," she told the Ottawa Citizen.
Yesterday, the manager again told Carla to come to him with her concerns first.
"I [told him] if I did see something unsafe, that I would just go to the police if I thought it was necessary," she told CBC News.
The manager didn't care for that response and terminated Carla on the spot.
In a statement, Wal-Mart Canada stood back the manager's decision, saying the company had clear guidelines in place concerning "pets in danger."
"We require our associates to follow these guidelines, which include reporting any safety concerns to a member of management, and engaging customers in a manner that is respectful," the statement continued.
Carla is the second employee in less than a month to be asked to leave after sounding the alarm about an animal found inside a car.
"I’m just worried that now two of us have sort of been let go, employees won’t say anything the next time and something’s going to happen," she said.