If it’s a desk drawer LOR (pretty much not allowed for officers anymore), there should be no record of it for any board to see. If they start a UIF on him and put the LOR in it, his response has to be included. Each of those things has a shelf life (not sure what USN’s policy is). Congressional inquiry, etc. is also a potential course of action, as well as the court of public opinion (media pressure works). If he gets separated before 20 (very difficult to do to an O-4), then it’s on to the BCMR.
Like a lawyer I’ve litigated against once said, he can drag it out “like a dawg with two broke legs” if he’s of a mind.
Saw it happen in the post-Vietnam Army RIF so it's not totally unknown. Saw an O-3 have to go back to the E-6 rank he held before OCS to complete his 20. It was an extremely ugly time and exacerbated by pusillanimous politicians like Billy Carter's brother.