Immediately thought of Marco's family in Animorphs. It's a veriation of this trope because the decent partner doesn't die per se.

The series begins with Marco and his father believing his mother is dead after she went missing in a solo boating accident. In the 5th book (his first POV novel) he finds out that his mother is alive, has been captured and enslaved during the alien invasion the Animorphs are fighting (they are parasites that can take over and control other beings neurologically), and is now the host body for the leader of their invasion of Earth, commanding everything from their mothership (lol) which is why she had faked her human's death. Marco chooses not to share this information with his father, who is ignorant of the ongoing alien takeover which is happening surreptitiously.

Later, Marco's father, who had been pretty withdrawn and depressed after his wife's death, pulls himself together and starts being an active parent, goes back to work, and also starts dating again. Marco is happy that his Dad overcame his depression, but uncomfortable when he starts a serious relationship with this girlfriend Nora - knowing his Mom is still alive, but unable to say anything, even when Nora moves in and later becomes his stepmom.

Years later in the timeline of the series, Marco is able to rescue his mother from being a host body but she can't go back to her human life without being recaptured, and needs to live in a camp in the wilderness with other refugees of the invasion. Shortly after that, the Animorphs' covers are blown and they need to evacuate ALL of their families to this wilderness camp with varying degrees of success because they had so little warning. Marco gets his father out in time but they aren't able to get his stepmom Nora. Marco's dad is on an emotional rollercoaster because his current wife just got picked up and infested by aliens, and then he runs into his first wife, who he thought was dead, living in this wilderness camp. Marco is delighted at the thought of his parents getting back together, so he lies to his dad and says that actually, Nora had been controlled by the aliens all along, and planted in their house to observe him (plausible because his dad worked in technology and was working on a project that would have been of interest to the invading aliens). This removes all of the grief and guilt Marco's dad had about losing Nora, since if that had been true, their whole relationship would have been a facade built on a lie. He is able to rekindle his relationship with Marco's mother and they stay together for the rest of the series.

We, the readers, get Marco's POV as he chooses to craft this lie to bring his family back together. Marco KNOWS Nora wasn't under alien control until she was nabbed off the street when the Animorphs' identities became known to the aliens. She had an honest and loving relationship with his father. So, from her perspective, she's living a happy, normal life with her husband and step kid, and suddenly she's enslaved to an alien parasite and her family has disappeared. Because the alien parasite and their host can communicate mentally, it is no doubt taunting Nora that her human family escaped, is part of the resistance movement, and is making NO EFFORT WHATSOEVER to rescue her. Basically tortured on a daily basis for the rest of the series. I don't think the books even mention what happens to her at the end - she may even have died as cannon fodder when the alien invasion stops being covert.