I agree with the need for documentation regarding the termination of Employer B's participation in the Plan. However, I am not certain about the reporting... though I may be misunderstanding the comments regarding reporting. It seems to me that Employer A would file the following:
For the year of the distribution, the Plan's Form 5500 indicating under Part IA that the Plan was a MEP, including a Schedule MEP that provides information for both Employer A and Employer B in Part II 2a. Note the 5500 is an annual disclosure form covering the PERIOD beginning ___ and ending ___. It is not based solely on the last day of the year. It seems to me no matter what day the distribution was made during that year, for at least one day in that year the Plan was a MEP. Also, the Schedule MEP Part II specifically asks for the % of contributions made "for the year" by the participating employers. So, it seems there would be information required to be disclosed for Employer B (though this may or may not be $0 depending on the facts). The other column in the Schedule MEP Part II requests account balance info relevant to each participating employer but it seems that would be filled in with $0 since this information would be the Employer B balance determined "at the end of the year".
Then, for the year after the distribution, the Plan's Form 5500 indicating under Part IA that the Plan is a single employer plan (and no Schedule MEP would be submitted). The omission of the Schedule MEP in that following year would indicate to the IRS that the employer not listed in the Form 5500 single employer filing that was listed in the prior year Schedule MEP is no longer a participating employer. (This seems to correlate to how the filing would be done if the Plan were still a MEP, i.e., the 5500 would be marked as being a MEP but no information concerning Employer B would be included on the Schedule MEP)).
I have not had to file a 5500 covering this type of situation before so I have not researched this and am just going off of what we have done when individual employers have withdrawn from MEPs we handle.