It doesn't really fit the diagnoses requirements, unless that person's passing was traumatic in itself and you were involved or around to witness it. Does this makes sense? There has to be some sense of fear of dying or nearly dying personally for the dx to fit.
Overwhelming grief is a very strong emotion though, and I'm sorry you are experiencing it.
However, if say that person that passed caused a person to be traumatized, what that someone might experience is a triggering of those intense emotions connected with the PTSD from the prior trauma, once the victimizer passes. Does this makes sense, also?