I don’t advise anyone here. Some fiduciaries might include in one’s reasoning:
If the plan’s administrator can do so without incurring an incremental expense, the administrator might send a demand letter asking the distributee to restore to the plan the overpaid amount.
If the plan’s trust collects a restored amount, the plan’s administrator might evaluate whether the employer had paid an amount to the plan under a mistake of fact and, if so, whether an amount (adjusted regarding loss or income) must or may be returned to the employer. See ERISA § 403(c)(2)(A)(i) and the governing documents’ provision.
Assuming customary provisions in the plan’s governing documents, a fiduciary might evaluate that it need not sue the distributee for a restoration of the overpaid amount if the fiduciary prudently finds that the value of the claim—discounted by the probability of getting the court’s judgment, and further discounted by the probability of collecting a judgment—is less than the court’s filing fee and the fees and expenses for the plan’s attorneys and their assistants.
Whatever is or isn’t done to correct this error, one hopes a plan’s independent qualified public accountant would see that the error is not material, and not even significant, in the plan’s financial statements.