"Thank you. Is the payoff amount wholly absent from the return? If so, may I have a reference or article to refer to if the client asks why?"

How about this perspective: That's already included in the numbers. The original purchase price is there. Whether or not it was paid with cash or by borrowing (mortgage) has nothing to do with anything. You're mixing in her Net Payoff with the sale details. Yes, it affects cash. No, it's not part of the gain or loss, because you already put her original purchase in there, and not listing only her cash down and principal payments all these years. There is no Reference. It's how the math works. Borrowing or not has nothing to do with the property. It has to do with if she gets all the sale proceeds, or part of it will pay off her loan.

You don't use Zillow for the price. You want her closing (HUD) document. Otherwise, how do you know the sale and the associated costs are correct? It's only 3-4 years ago. Your client should have their copy, or the title company has it.

Regular repairs don't really affect basis. One broken window or 3 water heaters over the year, don't count. All new windows would count. A new fence or sidewalk would count. A new furnace and water heater prior to sale ("fixing it up") would count.

We are all assuming she lived in it 2 of the recent 5 years as her primary residence, and not in assisted living or with relatives.