You should be able to do a substitute W-2 and put the numbers on there with an explanation as to what happened.  Have the client follow through with social security in 6 months to be sure it got applied to their account properly.  Pretty sure the gov't will take employee side, not sure if the employee can pay the employer side and show that on the return.  That might take a trip to the SS office directly.  However, that will flag the employer as under reporting, so think it through.