Another guy is pursuing your girlfriend even though he knows she's taken. How should she handle it?

Hypothetical question: You're in a committed relationship with a woman. She has had a male acquaintance for years whom she always thought was just a friend (and not even a close one at that). One day this friend tells your girlfriend that he wants a chance to date her. She responds by telling him only that she's "already in a relationship." However, this doesn't stop the guy. He continues to pursue her - asks her out to dinner, to the movies, etc., buys her little gifts here and there, posts semi-romantic things on Facebook directed at her, and even uses a photo of him and her as his profile picture. She doesn't respond to his advances, and she tells you she has no romantic interest in him at all. She also knows that you, as her boyfriend, are not at all comfortable with this guy's behavior. In all honesty and fairness, what do you think she should do?