From the Family Guy Addicts page, make sure you read this:
HOW DO I GET DIANE SIMMONS?
In order to unlock Diane, you need to get to the questline “HELL HATH NO FURY” Part 1. At that point you will then start digging up Diane Simmons. Once you repair her gravestone, it shows five days to unlock her. Once unlocked and in the game, she is yours to keep. If the time expires before you unlock her, she will disappear from the game and will not return.
To unlock her you will need to Brew the Electricity Ball and Chomps the Zombie in the Witch’s Cauldron. This will require a total of 400 Spirit Vials and 30 Terror Dog Bones. So you may want to get the Terror Dogs Spawning and some Vials stocked up before you repair her tombstone and start the timer.
So don't start clearing the graveyard until you at least have Ghostbusters Peter.