There are two concepts in play here.  Over-withheld taxes (employer screwed up, employer needs to refund) and Additional Medicare Tax (employer followed the rules, IRS needs to refund/count as a payment).

Since your box 5 amount is below the $200,000 threshold where Additional Medicare tax would kick-in, this one is clearly an employer screw up.  Figure out which employer made the mistake and your client will need to go back to that employer and get a refund.