If your parents don't have an ownership interest in the home (meaning their name(s) are on the deed, not just on the loan), then they can't deduct either the property taxes or the mortgage interest.
To qualify for the mortgage interest deduction, a taxpayer must have an ownership interest in the home.
https://www.irs.gov/publications/p936
The fact that they co-signed for the mortgage does not by itself qualify them to take the mortgage interest or property tax deduction.
If their name is on the deed, they can deduct the portion of the mortgage interest & property tax that they actually pay.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
