Thanks for your replying and meticulous answer.No -- first of all, the "where" here is a conjunction, not a relative pronoun. Neither "that" nor "who" would work, because what follows is a complete clause. (For example, we don't say *"A man who he is my friend.")
And I don't think it's colloquial, so much as that the author is considering the couple as a case or data point in this sentence rather than as people. This is where conj. II.10.b(a) in the OED: "In a or the case in which (often nearly = when 8); in the circumstances, position, or condition in which; in that respect or particular in which" (underlining added, as that phrase could be substituted for "where" in the example we have here).
Thank you for providing another way to understand "where"Yeah you lost me completely there Glen.
When I posted, I was thinking more of, for example,
In cases where ...
as an alternative to in cases in which
To me, this is a bit similar to saying
An accused person can state their case without fear of....
As an alternative to his or her case. It's colloquial.