But it is enough time to fix the issue. There is a potion you can buy with aspiration points that will reset your sims age to the start of elder, and the wishing well as well will do that. Plus there is the option of becoming a vamp assuming you have a vamp friend that can do the turning. Vamps do not die of old age. There are ways to turn the 'given enough time to say goodbye' to an honest spared or even immortality. But you must act fast a few hours to a few days isn't much time and the reaper isn't always so merciful.
By the way this isn't even going into the cheats by the way. "testingcheats" on then "cas.fulleditmode" then right clicking the elder sim, choosing edit in CAS will allow you to age them down thus avoiding the reaper.
Then you can invite the dead sim to move in as a ghost and get it back that way. And there are multiple ways to restore them at that point. So if you are not done with said sim, there are options. Just a thought.