Was a watching a mystery, one in which a major clue was the unknown perp being left-handed. So my wife said a particular actress on the show was obviously the villain. I asked how she knew:
Turned out they’d gone to high school together, been in a couple of plays. Ruth knew that actress was left-handed.
West Hunter 
Left-handed people are sinister.
Beat me to it, dammit.
The princess bridge is near impossible then
The princess bride. Damn the auto correct
There’s “breaking the fourth wall”, as when an actor addresses the audience directly. It messes with the suspension of disbelief in the minds of the audience which is necessary for immersing themselves in the story. It is usually done for comic effect. Your case sounds like a variation on that, in reverse. -An audience member breaks the fourth wall, letting reality intrude into the fiction.
https://tvtropes.org/pmwiki/pmwiki.php/Main/BreakingtheFourthWall
Synchronicity?
Your wife’s friend probably answered a casting call for a left-handed actress.
My first thought was “That was serendipitous”.
Side channel attack.
Inside knowledge? Meta-reasoning?
Analogical Leap. An interesting person I never heard of talks about it WRT programming here: https://argumatronic.com/posts/2018-09-02-effective-metaphor.html “Metaphors don’t scale very well.”
Duckduckgo tells me that Maurice Blondel used the concept as well.
The weakness is that using the term “analogy” to describe the connection between the actress and the character isn’t quite solid.
Leap of recognition might also work. Ruth recognised which trivial detail was actually important.
I saw a few Harold Lloyd movies before realizing he had a prosthetic hand.
Also, knew a guy who had a finger reattached. Never noticed until he mentioned it, even though it was really obvious.
The actor who played “Scotty” on ST:TOS was missing fingers on one hand (WWII, I believe). The show decided to conceal this fact, always shooting him in such a way that you can’t see the hand in question.
I watched all episodes multiple times, and the movies, and never noticed that.
Inferring. (On all the data available.)
^this
We watched a ‘tec show last night. Among the potential suspects were two played by well-known actors. So whodunnit? The answer was neither: the clever director had misdirected our suspicions by the needless casting of well-known actors.
Illogical. A right-handed actor can play a left-handed character.
Exactly my thought. But I learned a bit, e.g. about breaking the fourth wall, while reading the comments. This as a common experience for me on West Hunter.
Why stop there?
There can be more than one left-handed person.
The clue could have been wrong.
Greg’s wife could have a faulty memory.
The villian might not exist, it might just be hysteria.
The actors might be aliens with no concept of handedness.
etc.
Because one is good enough for a mic drop.
I’d say the words are “coincidence” (that your wife knew the woman in real life), and “inference” (that she decided the producers would use someone really left-handed for that role).
Category error.
“Yosu-miru”
“privileged knowledge”
Extratextual reference.
Synistergistic
Coincidence
meta inference
Artistic gnosis.
I married and divorced a left-handed woman. It ran in the family, along with some other undesirable traits, such as narcissism and sociopathy. I distrust lefties.
I have always rolled my eyes at mysteries in which a detective says something like, “The victim was obviously bludgeoned to death by a left-handed killer.” More than a few lsouthpaws, myself included, write with our left hand but reserve our right hand for tasks such as using a screwdriver or brandishing a fireplace poker.
Information asymmetry.
It occurs to me that you said “word,” but I used two words.
Inside information.
If I were your wife I would have kept quiet about knowing the actress and attributed my guess to an uncanny intuition that came from, I know not where.
I watch a whodunnit? last week. That’s all.