I've had problems with PEO's getting payroll issues wrong.
You should contact your client and let him know what the situation is. Tell your client that unless the PEO issues a W-2C, you will have to file the returns based on the incorrect W-2, then amend the return, for which you will have to charge him, when the W-2C is issued. When the client learns that he will be overpaying tax because of the PEO's mistake, plus additional prep fees, he will swing into action and hopefully put enough pressure on them to issue the W-2C next week.
I would also prepare a pro-forma W-2C and circle the changes, so the client can send it to the PEO and the dummies on the other end will know exactly what to do.
The 941X is not so urgent, but the client should make sure that it gets filed.