I would verify with the individual that you have properly recorded the TIN. If the number is accurate, I would complete a SAR for identity theft.
I recommend contacting your bank counsel as to how to address issue with employer. Unfortunately, the Bank now has constructive evidence that this person may be an illegal alien and you also have reason to conclude that you may not know the true identity of this individual. I can't think of any reg or law that compels us to report or prevents us from reporting the circumstances to the account owner, but that isn't an action I would want to take without counsel guidance.
Recent announcement by Homeland Security regarding amendments to preclude hiring illegal aliens may help us with these type issues down the road, but that doesn't help you today.
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Life without Jesus is like an unsharpened pencil - it has no point.