Y do guys think that their X-gf is still there property and have to tell them everything??

My friend i am staying with at the moment her x is after her as she has moved on with her life ad is dating someone else..he rocks up here and abuses her ad then trys to go for her new bf..Why do guys have a problem of its ova and y can't they just leave there x alone. She has told him face to face it was neva anything more then friends and they neva had nothing..he just can't seem to get over my friend...What can we do to get rid of him?????