First the balance of the mortgage is immaterial to the taxable gain on the return.
Next the calculations of the gain are not as simple as you think. Mom should have been depreciating the rental property all the years it was a rental for profit. If no depreciation was properly taken then RUN to a local tax pro to get this issue corrected on the return ... this is NOT a DIY project.
https://ttlc.intuit.com/questions/1933977-i-sold-my-rental-property-how-do-i-report-that?jump_to=ans...
