Money maybe, or quite possibly he doesn't want there to be any chance, even the remotest, that she might recover to the point of some sort of communication. (I think he caused her 'accident'.)
My suspicion is this: that the husband knows that whenever she dies, there will be at least a book and most probably a movie that will reap him untold monies. Not only has he a girlfriend, but TWO children with that woman. He is still legally married to Terry, and I think the parents made a big mistake in not trying to get her divorced, somehow, from this monster many years ago.
Without some divine intervention, in some untold and fantastic manner, I fear that this will have a very bad ending.