I assume that even if she did ultimately vote for Biden, it was reluctantly and secretively. Clearly it would NOT be something she would tell her husband.
I don't think it is important who she voted for, as a public figure she likely was far too conflicted to be an opinion leader either way. Not to mention, she likely does not have the cache or gravitas to be seen by almost anyone as a voice to listen to. Her 2019/2020 comments on Trump were very negative, but were not on political issues, but on Trump's despicable attacks on her father. Before marrying a right wing man, that might have been enough for her to vote for Biden, a man she knew to be kind.
The precedent there was that she did publicly say in 2004 that she voted for Kerry, because his support and kindness to her father when McCain was attacked when they were on the Vietnam committee contrasted to Bush's 2000 attacks on her dad. Kerry was to the left of Biden, so I doubt she was politically closer and both are good men. Bush's letting Rove attack McCain as he did was as bad as Trump's many attacks.
I have no idea if she left that office unvoted, voted for a third party, wrote someone in -- or voted Biden. If she did, there is no reason to tell her husband. By the way, what does it say that the husband ENTHUSIASTICALLY supported a man who repeatedly trashed his wife's recently deceased father?
I am willing to be proved wrong, but I doubt that Megan McCain is a good right wing surrogate. She is chaotic and petulant and has no real claim to any expertise on anything - other than John McCain. Even on that, many others likely know McCain, from a political vantage far better and are not blinded by hero worship. As long as Megan IS the "conservative" on the show, they are disadvantaged.