Symbolism, Imagery, Allegory
James, who was both a prolific writer of ghost stories and a staunch Victorian, dramatically equates divorce with death early on in Maisie, when the narrator refers to Ida as Beale's "late wife" (Preface.1).
This doesn't mean that Ida Farange is literally dead, but this may be a clue that she, like others in little Maisie's world, is dead inside. And the idea that James's characters are undead—not full-blown zombies but somehow half-alive—keeps coming back to haunt them later, as well.
This makes Maisie's constant energy and her unshakeable willingness to face challenges of all kinds that much more remarkable. Lots of adults may be half-dead, but Maisie is very much alive.
James is also parading his Victorian sensibilities with this comment: he's talking about how the institution of marriage is dead. Divorcees aren't just dead inside; they're evidence that society itself is crumbling. James here is throwing up his hands and saying, "Divorce! What next? Ladies wearing trousers?!"
Although he does have a point, with these characters. Ida and Beale aren't merely people who fell in love, tried to make a go of it, and failed. They're serial monogamists and flit from relationship to relationship without any sense of commitment, and they don't care whose hearts get trampled on as long as they continue to have fun and rack up notches on their respective bedposts. They don't even care if young Maisie is left in the lurch. And that's messed up by any standard.
Much like the spate of zombie fiction in the 2000s and early 2010s was a reflection of some pretty dire circumstances, Henry James's comments on divorcees being "undead" is his way of giving us some oblique social commentary. James? Being oblique? You don't say.