There are many reasons why a girl would choose to be friends with guys. There's nothing wrong with that in and of itself. However, your girlfriend is in a position in which she knows that staying friends with this guy is hurting him, hurting you and hurting your relationship. Question motives. Why would she choose to do this? Likely, she is insecure as most young girls are and enjoys the feeling of flattery of having someone interested even if she does not intend to persue it. Again, it's not unusual to feel this way. It is, however, unacceptable. Only a bitch would be so selfish to not give your friend enough space to get over her and move on with his life. This isn't the kind of person you want to be with.
Secondly, do not talk to your friend. Your friend has betrayed you and it's clear that even you aren't interested in being his friend. There's nothing to work out and any attempt at communication of the traditional sort could only be twisted and used against you somehow. If he won't leave your girlfriend alone after she actually tells him no (and be sure she's actually done this as you say she has), then kick his ass. Some lessons have to be taught the hard way. This is a common one and an ass kicking is kinder to him than what his fate may be if he makes a lifestyle of convincing women to leave or cheat on their boyfriends.
In summary: dump her, kick his ass.