Relative has overstayed in Canada for 20+ years

I have a relative (British citizen) who moved to Canada 25 years ago, but just entered the country as a tourist. They had a Canadian spouse but never bothered to regularise their status in Canada, which I assume would have been fairly easy given their spouse was a Canadian citizen.

Fast forward to 2025, their spouse has passed away and they are moving back to the UK. Will their overstay be picked up anywhere (I am aware Canada doesn’t have exit checks at the border) and if so, what would the likely consequences be?