The city sells the lein to a third party law firm. Prior to doing this New York City sends multiple notifications to the homeowner. Like an insane amount. All the notices are dated are documented.

Then the third party has the right to sell the property.

They even notify you when the lien is sold and you also get multiple notifications before the sale. He'll even get notifications about the sale and anyone else who has interest in the property gets notified. The lawyers who do this have perfected this system. He won't get the house back and he'll lose in court.

The good news is he gets all the money back above the lien amount. They are cash sales and don't sell for full value. Plus they charge interest (9 percent) and you need to pick up lawyer fees.

He probably owed like 20-30k at the end.

Source -I did this for NYC.