What you've been told is correct. By filing separately that will keep the loan payments lower. But it will stretch out the time it takes to pay off that loan, and she will pay significantly more in interest over the long term. Additionally, when a married couple files separate returns, that automatically disqualifies them both for deductions and credits they would otherwise qualify for if they filed joint. So by filing separate that means you pay more in interest over the long term, and you potentially pay more taxes each year you file separate.
Whereas if you file joint, while the loan payments will be higher you will pay the loan off sooner meaning you pay less interest over the shortened life of the loan, and you get to take those deductions and credits you qualify for filing joint, thus paying less taxes.
So what you don't pay to the lender in the form of more interest, you will pay to the IRS in the form of higher taxes.
Being that 2018 will be your first year filing joint, that means you will be starting your first joint return from scratch. So I would highly recommend you purchase the desktop version of TurboTax and install it on your computer. Then you can work it both ways to see what difference there is and decide which way you want to go with this.
1) File separate and spend the rest of your lives "working for the man" to pay the lender more interest an the IRS higher taxes, or
2) File jointly, pay less taxes, higher loan payments resulting in less interest on the loan and get this 800 lb gorilla out of the room and off the property as fast as you can.
