Very, very, very unimpressed with this one. It was a weak, overused idea - both the person disintigrating instantly because they'd clung to life too long - and the stranger pretending to be the loved one of a delirious, moribund family member (and yes, Star Trek already did it too, Voyager to be specific). I held out to the last, hoping for the twist which would save things - and, alas, it never came.
The language was good, but had no sense of timing or pacing. Scenes which should have been punchy instead dragged on. When a clipped fragment appended with a colon like ":Randy, her son." would have sufficed there was instead a looooooong sentence that conveyed the same information. More attention to the changing in pacing conveyed by sentence length would have helped this one. Well... it would have helpled the prose, anyway. The idea and plot would have required much more to resolve.
Another sad fact about such well-treaded ground is that it invites comparison to the masterworks of said niche. In this case, "Cool Air" by H. P. Lovecraft and "The Novel of the White Powder" by Arthur Machen - both of which pull off a similar idea, but with much more terror.