I believe this person is mixing up a couple of historical figures. Firstly they are most likely thinking of president Woodrow Wilson who suffered a major stroke in his last year in office, effectively leaving him unable to fulfill the roles of the office. This was kept largely secret with many of the important behind the scenes decisions being made by his wife, such as which documents he read and who was allowed to speak with him. Some have called her the first female president for this reason, whether or not you subscribe to this logic is up to you. However Wilson was certainly not "sundowning" as it were, mentally speaking he was assessed as being clear headed but physically enfeebled and emotionally unstable.

Secondly they may be thinking of Ronald Reagan who was diagnosed with Alzheimers in 1994 (six years after leaving office). There are many who believe that Reagan was already suffering from Alzheimers in his later years in office though there are many in the medical and historical field who refute this. Historians generally do not consider Reagan to have been suffering Dementia or Alzheimer's during his time in office. Though perhaps part of the reason why this is a commonly held belief is because of Reagans public persona, particularly his actions when targeted by scandal. Famously, to avoid implicating himself in the Iran-Contra affair Reagan would repeatedly testify that he "did not recall" whether or not he had attended certain meetings or spoken to certain people. If taking this testimony at face value it would seem that he had severe memory and mental incapacities. If taking this testimony more cynically, this is a legal tactic to avoid blame without outright denying a claim. Certainly Reagan was not above at least appearing to have questionable mental capacities, though exactly how astute he actually was behind the scenes is another story.

As for Nixon, to my knowledge Nixon was never diagnosed with Alzheimer's and certainly retained a high level of function throughout his time in office. He continued active work in politics as an elder statesman type for years after leaving office. Simply put: while I am by no means an expert on Nixon, I've never seen or read anything before this post to suggest that he was losing his mental abilities either during his presidency or even after it.

So I think they're mixing up traits and facts about Wilson and Reagan, and then falsely believing that both of these cases were actually just one case in the form of Nixon. Basically they forgot to check their notes.