You can enter the interest paid only if you are also liable for the debt.
If you are on the loan, and not on the 1098 then make sure the total interest deducted by the two of you only totals up to 100% of the total interest paid. If you are not on the loan then you cannot deduct any interest.
If you are on the loan see below for additional detail:
https://ttlc.intuit.com/replies/5698669