I'm not a tax accountant, so I'm not going to say definitively that this is allowed in a housemate situation. But it would definitely be if the property was a duplex.

The fractional rental, if legal, gives your partner advantages that others have not mentioned. These won't turn the mortgage into a profit center, but nearly.

Your partner may have the opportunity to deduct a huge variety of home expenses that homeowners can't normally deduct. Or rather, half of those expenses.

Depreciation: This only applies to the value of the property improvements not the bare land, but in a rental property, 1 / 27.5 (about 3.6%) of the improvements' value is deductible every year. (or half of them in a half-rental).

Suppose improvement value is $150k, that depreciates $5454 per year or $454/month. Suppose another $150/month goes into repairs, so $600/month; she deducts half or $300. In a 30% bracket that provides $90 in tax refund.

While your partner may pay a $1000 mortgage, that is not the same as rent. Not all of that payment is interest; some of it is principal, and that pays down the note, increasing your partner's equity in the house. You both pay $500 toward the mortgage, but she (unlike you) gets $200 back in paper equity. Note that you cannot buy a hamburger with paper equity. So it nets out to only $300 in cost to your partner. With the afore-armwaved $90 in tax refund from depreciation and maintenance deductions, your partner's net cost to live there is a mere $210, down from $1000.