My bro-n-law worked w a fake ssn for 4yrs. He is now legal, married w a legal ssn. How should he file taxes?

Use the new number. IRS does not care about immigration status as long as you pay your income tax. You think IRS don't know you are using a fake SSN. They don't care as long as you file your tax. IRS would rather spend tax payer dollars to go after people/corporations for not filing multimillion dollar tax return than to go after a person using a fake SSN to file a $50,000 return. I am surprised that you even have to ask this question considering what you happening in the financial world. IRS now are going after people who make millions in bonuses and they don't care about small fries like us cause OB need us to pay tax to cover his Trillion Dollar spending.