Ugh.  Good luck.  Either way the employer (or at least AN employer) screwed something up.  If it was a stock sale they should have continued with the YTD SS wages.  If it was an asset sale they should have reported the second W2 under a different EIN.

Looks like F843 wants an employer statement attached.  Good luck getting that (but I think you have to try!)  Maybe you (or client) can escalate far enough up the chain of command to reach someone smart enough to realize that they likely paid way too much in employER SS tax too and the clock is ticking to amend those payroll returns.  It's really in their best interest to fix it on their end without involving the IRS.