Her paying off your mortgage amounts to a gift from her to you. Gifts are not taxable, or reportable, to the person receiving the gift (you) or deductible by the person giving the gift. So, it doesn't matter whether she pays the mortgage directly or gives the money to you to pay it. Any part of the payment that goes to paying interest is deductible to you.
If the gift is more than $14,000, the giver is required to file a Gift tax return (form 709). The gift tax filing is separate from income tax. "Gift Tax" is somewhat of a misnomer. Even though a gift tax return may be required, very few people ever actually pay federal gift tax. The purpose of the gift tax return is usually only to document a reduction in the allowable estate tax exemption.
See <a rel="nofollow" target="_blank" href="
https://turbotax.intuit.com/tax-tools/tax-tips/Tax-Planning-and-Checklists/The-Gift-Tax-Made-Simple/...>
