Yes, it's any 5 year period. If you have been PR for less than 5 years, you can not be outside Canada for more than 1095 days in your first 5 years. Obviously you may have been living outside Canada before you became PR so that doesn't count.
After you have been PR for 5 years, it's always the last 5 years counting from today. It's your obligation to make sure that you are not gone more than 1095 days in that period or you risk losing your PR. The risk is less when you have a valid PR card but the border guards can stop you at any time on re-entry if they suspect you don't meet the residency requirements anymore and give you a time limit to prove it.
It's tricky as it's almost 5 years since they have been PR, they have lived in Canada for 2 years and a few months, so they can keep their immigrant status, I wonder if the paper of landed immigrant would still work as they returned, even if their PR card expired when they are outside of Canada?