I'm guessing graveyard sex is for people who don't yet have their own houses, apartments or rooms in which to enjoy privacy and for whom parks are too full of living passers-by? Something about a cemetery doesn't exactly spell celebration of life, although birds, flowers, and trees are lovely components of many burial areas. I'm thinking your friend's sister needs to take up camping and move a little further out into nature if she wants to get her groove on outside.