He can be prosecuted for identity theft if the number actually belongs to someone else, and possibly fraud even if it didn't. Also, it can be grounds for deportation if the person is not a U.S. citizen.
Keep in mind. There is some poor guy in Oklahoma trying to explain to the IRS how he never made this money in Arizona that somebody put on his SSN. It is not a victimless crime and never believe it is. You are possibly costing somebody a lot of money to defend himself against income he never earned in a state he never lived in.
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Yes. It is a crime.
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