In all honesty it would be uncomfortable knowing that there was a cadaver on board, but to sit next to one! Was he really next to it? If they upgraded a son and daughter it sounds like they provided three seats, it's not difficult, especially in a first class cabin, to put as much space between the deceased and fare paying passengers.
As to saying the first class cabin is the quietest seems odd, it's not like anything is going to disturb her, but with limited choices at the time (yes, there probably should be a small area that can be used for this type of storage, but for this flight there wasn't) they did the best they could.