@JGTMoney
The taxpayer who claims the deduction must be legally entitled and also be the person who actually paid the expenses. As a practical matter, if you claim half and your GF claims 100% of the interest and taxes, the IRS will probably catch it by matching the interest claimed to the 1098 issued by the bank. In that case, you will both get letters, and you will need to be able to prove your case that you paid half the expenses. Good luck.
