No, you do not have to file MFJ, and the wife's action indicates that is something she does not want.  Go ahead and file your client MFS (wife is obviously not your client anymore), and don't lose any sleep over what the wife did.  Her SSN goes on his return but IRS has better things to do than figure out her correct filing status.  

If they get back together, see if it lasts more than a few months, and then file a joint 1040X if they want and if it saves any tax.