Yes, the soul of a victim of diablerie can persist after the event, though this situation may not be able to leverage that.
It doesn't depend on the diablerist's mercy. It's a function of the strength of the victim's soul:
A few rumors speak of diablerists displaying certain
mannerisms of their late victims, particularly if the victims
were of great psychic fortitude (Willpower 10) and
of much stronger Blood than their murderers. If this is
true, and the soul of a particularly mighty undead can
manifest in the body of its killer, the implications are
frightening, particularly in light of the Jyhad. (V20 Core Rulebook, page 295)
Emphasis mine. This is yet another instance where the rulebook is unclear on purpose to heighten mystery. But it's often the case that things the books suggest to be true in this way later turn out to have occurred in the canon.
I'm not aware of explicit mechanics for how this process unfolds. That means it's the Storyteller's remit, in which case the details can be whatever you'd like.
We do have at least one explicit, claimed example, though:
During all of this, our greatest loss was one of our own
elders. Goratrix, whose work had granted us immortality,
defected to the Anarchs. He claimed the leadership of
Clan Tremere was corrupt. He spread a ridiculous rumor
that our founder had been possessed by Saulot, and been
forced to move his real spirit to Goratrix’s body. (V20 Lore of the Clans, page 210)
-
I don't know what the plans are for the meta plot in V20, but in Revised this story was explicitly true. (I'm having trouble finding the exact citation I remember, but the Time of Judgement books make many references to this, particularly Gehenna)
Souls don't necessarily endure diablerie, but it's not totally clear what happens to them if they fail to do so:
Lastly, for Camarilla vampires and others who adhere
to the way of Humanity, there is the loss of Humanity
to consider. Diablerie is worse than murder: The Amaranth
literally absorbs the victim’s soul, destroying any
chance of the victim finding peace in the afterlife. (V20 Core Rulebook, page 295)
It's possible that literally absorbing the soul of someone destroys it, but it's just as possible that the absorption maintains the soul as it is subsumed into the diablerist, and also just as possible that the soul is irretrievably altered or destroyed in the process.
If the soul of the victim isn't "strong enough" to resist (whatever that means, mechanically), it may be gone for good. For that reason, the wife's soul still existing in some retrievable way could require something close to a stalemate between herself and her husband's childe. That state of affairs may not be possible if the wife diablerized the childe and the childe won that struggle (to take over the wife's body).
The precise mechanics of what diablerie means and how it works are unclear, so that isn't necessarily an impossible obstacle. However, the overall rules about diablerie and its representation in-game suggest that the act should not be convenient or safely predictable. I would be cautious about allowing diablerie to solve problems, especially if the goal is to do so in a straightforward way.
Conversely, if your conclusion is that the wife's soul remains intact in her body, getting assistance from the Tremere or especially the Giovanni would be a more clearly RAW- and canon-friendly way to attempt to recover and/or preserve the wife's soul.
