If a mortgage is required to purchase the property and there is a co-borrowing relationship established, a decision will also need to be made regarding who takes the mortgage interest deduction on their tax return. According to the IRS, the deduction can only be taken by a person who pays the mortgage and owns (or jointly owns) the property.
If a parent and child both own the property and are co-borrowers on the mortgage, but only the child pays the mortgage and property taxes, they would get to take the deduction. Interestingly, it also means that if the parent owns the property, but the child makes the mortgage payment each month, then neither the parent nor the child qualifies for the interest deduction. If the interest is split between parent and child, both can claim the mortgage interest deduction, but it should be split based on what was actually paid by each owner during the year. This typically complicates the tax filings as it is common for only the first person listed on the loan to receive Form 1098 (below) from the mortgage lender.
A supplemental statement included with each tax return explaining the split of the mortgage interest would be prudent. This would be another consideration that could be fully defined within an agreement regarding the property.
https://www.irs.gov/faqs/itemized-deductions-standard-deduction/
other-deduction-questions/other-deduction-questions-2