I would suspect that in court the Plan Sponsor and/or Plan would prevail over the fraudulent claim for benefits from the person not entitled to those benefits but would likely need to determine if the cost of pursuing it is worth the recovery of the funds.
If they can't recover the funds then yes they would have to restore them.
If I were advising the employer, I would advise it to pay the money into the plan, with some reasonable interest factor. Probably a breach of fiduciary duty in causing the money to be paid to the wrong person (lack of sufficient internal controls, etc.). The employer can then attempt to collect from the individual to whom it was paid, but the employer should face up to the fact that it may have to suffer the loss if it can't collect.