certified cheque

This is US law, maybe Canada has different rules, but:

https://www.sapling.com/8611754/do-void-certified-check

But the New York Credit Union Association cites §3-403(2) of the UCC. This section indicates that payment can be stopped on a certified check if ‌90 days‌ have passed since it was issued ‌and‌ the check has been lost, destroyed or stolen. The remitter would have to submit an affidavit to the bank or credit union in this case and request a stop payment order in writing.

The CP admitting that they lost the check should suffice as evidence that the cheque has been "lost, destroyed or stolen".