CANADA: WORKING WHILST WAITING FOR APPROVED POST-GRADUATION WORK PERMIT TO BE MAILED.

Hi. Please I kindly need some advise on behalf of my sister.

She received her Canadian Post-graduation work permit approved online on the 21st of February 2022. Then she was told she would receive it in mail in 6 weeks time.

She was working after she finished Uni and continued to do so whilst waiting for the result. Her study permit was valid till a week before the PGWP was approved.

As she waits for the approved PGWP via mail, what can she show her employer that she can continue working and her PGWP was approved?

Or can she only work after she has received the PGWP thorough mail?