Neither. There is an age difference, sure, but an 80 year old woman is a well-lived human adult who can make her own choices. If it were an 80 year old female elf, then possible. Not sure what the age of consent is in elvish customs, or the age of emotional maturity. As for robbing the grave, no, she’s not dead. Robbing the cradle implies young life inside the cradle and there is nothing living in a grave. Now, if the elf provided the woman with prolonged life through elvish ways, then technically, he’s robbing the grave.