I don’t know what kind of relationship they have but if he said "Ok mom, I’ll buy you this house as long as you pay the property tax (which was probably, lets say, $600 a month on the high side) and then she decided to stop paying it…what’s he supposed to do. If he doesn’t have the money to cover a few years of back taxes ISTM that’s his only option.
I suppose if the wanted to be ‘nice’ he could sell the house, pay the back taxes and give her enough money to buy a new house but make sure the new house is totally in her name so when she stops paying the taxes on that house it’s her problem and not his. That was his mistake. When he bought her the house he had a career and an income and life was good.
I wonder who was paying the property tax at the beginning, him or her.
I wonder, and this I think is the important thing, if he ever said “Mom, I can’t afford this, you’re going to have to find a smaller place/move in to sister’s house/get your own apartment/get a job etc” and what she said. Basically, at some point, regardless of who was initially responsible for making the tax payments, the city came to him for the back taxes. He told mom he couldn’t afford them. What did she do? Did she work with him to figure something out or did she sit tight and tell him to deal with it because she wasn’t going to move out of her house.
ETA, hopefully that all made sense, I was thinking about it as I was typing it. The more I typed the more questions came to mind.