I nominate this for the "Ultimate Trick the IRS Computer Maneuver Award" of this tax season.  @IRonMaN is right that a paper return with attached explanation may be the best solution.  Also, the quickest solution, even counting the time needed to send IRS another copy in 18 months. 

This payment should obviously have been reported on a W-2.  (If sick-pay insurers can issue W-2s to non-employees, why can't this benefit company?)  But what do you do when your client doesn't receive a W-2?  That might not be covered the first day of Income Tax 101, but it certainly will be before the last. 

Form 4852.  Amount from the NEC on Lines 7a and 7c.  Nothing on Line b.  

IRS Instructions for 7c—Medicare wages and tips. Enter your total wages and tips subject to Medicare tax. This amount may be required to be entered on Form 8959, Additional Medicare Tax

If you can't do that easily, remember that the employer is responsible when taxes are not withheld, and IRS may have already come after them as far as you know.  Maybe set aside 1.45% in case anyone asks and it doesn't fit easily on the return.