What should a guarantor do who guaranteed a person for a loan that turned out to be usurious, while he was unaware of that, especially since the guarantor is a government employee and guarantees repayment if the borrower fails to pay?

Whoever guaranteed a borrower with interest, unknowingly, bears no sin. He must withdraw his guarantee once he knows, because guaranteeing an interest-based loan is Islamically invalid; meaning, it is not binding and its effects are not produced.