The legal requirement is to provide the required information within 28 days beginning with (or 27 days from) the date of service of the notice requiring the information. Any such notice, if properly posted, is deemed served (delivered) 2 working days after posting, unless and until the contrary is proven (on the balance of probabilities).
If the first notice was not served (as a finding of fact by the court), then the next question would seem to be whether the reminder is capable of constituting an s. 172 requirement in and of itself - in which case the 28 day clock would start ticking on Match 19th, assuming that it was properly posted by first class post on March 17th and there was no way to rebut the presumption of service.
Where a lawful s. 172 requirement has already been made, a reminder has no status in law (albeit from Scottish case law), other than a nicety either reminding the addressee of the requirement, or offering another bite at the cherry. Where no lawful s. 172 requirement has been made, a notice intended as a reminder would constitute *the* requirement, if it contains a requirement under s. 172. I avoid using the term "original requirement", because it is misleading. There can legally only be one requirement
Assuming that she is the RK of the vehicle in question, the original NIP would have had to have been posted between 9th -18th Feb and deemed served 11th - 20th Feb - with the 28 day clock expiring on 10th - 19th March, so a reminder posted on 17th March by first class post would be deemed to be served either on the last day that the response was due to be received, or 1 - 8 days after the 28 days had already expired. Which would make the reminder seemingly somewhat pointless.
N.B. If you must post on behalf of others, please at least try to avoid cherry-picking the parts of the notices that you consider relevant or curious. The purpose of this forum is to help motorists facing motoring allegations, not to help their mum interpret a poorly written phrase from, a civilian department within a police force, in isolation.