While there is no guarantee that CBSA would permit re-entry, CIC usually doesn't complain about PAs leaving for a short time. No one seems to know the exact maximum, but less than a month is typically accepted.
If the Sponsor is a PR and not a citizen, they cannot leave Canada until the file is complete. If they do, the file will be rejected.
Any visas or permits are NOT invalidated UNTIL COPR is signed by both parties. So, no issue.
If they contact you via email, check your email while on vacation. If you get a landing invite, you can email your return date, and they will delay your landing.