The Answer is B, one year before the time of scheduled payment, unless Plan language is unduly restrictive. Were that not the case it would not be possible for separated employees to make later postponement elections, and I see that all the time with installment elections when each payment is deemed a separate payment.
I am assuming that the initial election was for a separation from service payment or for the first to occur of the two, fixed date or separation. That plan language allowed for two different payment events, separation from service OR fixed date.
Check the election he made. Which was it? Then double check the plan's postponement language. 0ne year before payment date is all you need.
(Not legal advice, etc.,etc.)