Each of you should deduct the amount of mortgage interest that you actually paid. In preparing your tax return, you would indicate that the amount of interest you are reporting is not what was sent on your 1098. See screenshot below.
You do not need to attach a written explanation for the difference. You should have documentation to support your deduction kept with your tax papers.
Your girlfriend would indicate that she did not receive a 1098 and enter her share of the mortgage interest. See screenshot below.
