Assuming your client:
1)does not work there any longer & doesn't intend to ever work there again, or
2) if he still works there is prepared to get fired,
here's what I believe you should do:
1) Forget the 4852 - it isn't relevant
2) Prepare the 4137 & strike through the wording on line 1 - enter "Wages" on the dotted lines. The 4137 will calculate the employee portion of the SocSec/FICA tax and add it to his liability, but will omit the employer portion.
3) Prepare an SS-8 with employee name & SS# and all employer info. I prefer to have the client fill in the actual answers to the questions in his own words, but that's just me. (The main determinant is what the employee understood his employment situation to be, so I think it's important that he be able to explain it in his own words. I don't want to be accused of misrepresenting the situation if it's questioned).
4) Mark one copy of the SS-8 as "For Information Purposes Only" and attach to the return.
5) Mail the original of the SS-8 to the IRS office where it is required to be filed.
The above will save your client about $800 (the employer's portion of the SocSec/FICA tax) minus whatever you charge him for doing the extra paperwork. The IRS will probably not pursue it, but if they do the employer may get very upset with your client. (That's the reason for my opening comments). The other possibility is that the IRS will pursue it and disagree with your client, billing him the $800 plus interest a year or two down the road. I think this is the least likely outcome, but you should warn the client that it could happen and that you won't be responsible for the P&I if that should come to pass.
I've done a few of these and never heard another word about them, but one never knows what the future holds.