He's dead, Jim
One method: A 17th+-level wizard could sell her services as an eternal caretaker (if you have to ask how much she charges, you can't afford her). She takes your victim, kills him, imprisons the soul in a Soul Bind spell, and keeps the gem on her person. The wizard casts Mind Blank every morning, so she is continually immune to all information/divination magic. Your victim is dead, cannot be resurrected, and even wish/miracle will give NO clues or help in the matter.