To deduct mortgage interest, the taxpayer must be a person who is obligated to pay interest (by being named on the mortgage) and be a person who actually pays interest. The deduction is limited to the amount of interest they pay.
However, note that property taxes may only be deducted by someone against whom the taxes are assessed, and be the person who pays the taxes. Generally, your father won't be able to deduct property taxes unless his name is on the deed of the home as co-owner, as well as paying part of the mortgage payment.
Now, there may be some leeway in how you divide up the finances. Suppose the mortgage payment is $4000 per month. That might be $3000 for interest, $600 for property taxes and insurance (escrow items), and $400 to reduce the principal. In that case, it would be possible to say that your father is paying half the interest, but is not paying toward the taxes or principal. (Remember that even if your father covered the entire mortgage payment, he can't deduct the taxes unless he is on the deed, not just a co-signer.)
Ultimately, is the extra deduction he might be claiming, worth fighting over and risking the house?
