If the accusations were provably false, your friend could hiring an attorney and filing a lawsuit against his accuser. But this is probably not a good use of your friend's time or money because he will have a hard time proving that he suffered any damage from this accusation. Given that his employer investigated the issue and found no wrongdoing, it sounds like your friend did not suffer any appreciable harm.
Disclaimer: This answer is for informational purposes only and should not be treated as legal advice. This answer does not create an attorney-client relationship.
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