Someone I know bought a property in the 70s to use as a second home. They did this and then turned it in to a rental, self-managed, after a number of years. They never took depreciation on the property for whatever reason. The person is now selling the property for about $100k. The property has been vacant for the past 2 years. The only income this person receives is about $800/month Social Security (this person is in their 90s).

The person doesn't remember how much they paid for the property (either the realtor or the person will have to go to the assessor's office to see if there is some document on file stating how much was paid). Let's just go off paying $10k to make numbers easy. So, that's a $90k gain. Will that be added to the social security amount to calculate Capital Gains or is it dependent only on the ordinary income, is social security considered part of ordinary income to determine capital gains?

I'm trying to ballpark what might be owed for 2019 taxes for this person. Does it matter that the person never took depreciation, or much of any tax breaks for the property? The person usually never really owed any tax due to such low income in general.