Sounds to me (from your point ofview, at least) like she is seeing someone other than you as more suitable mate material. Being sensitive and cool about this is not going to help, and frankly maybe nothing will.

If you are absolutely sold on this girl, then my advice would be to 1: let her know that you do not want, and will not accept her spending time with this "friend". Obviously your only recourse is to dump her if she pesrsists, so make it clear this is exactly what you will do. She probably will say you are overreacting, and that you are being controlling. Respond that you are not comfortable with her relationship with this person, you do not like him, and that if she persisits it will be clear to you that you will leave. This -might- work....but probably not, because she obviously thinks you are a pushover, so probably will not believe you.

2: If she persists, leave her immediately. Tell her to call you when she grows up. DO NOT CONTACT HER. Go about your business, be sure to get out with some female company where she or her friends will see you. If you see her with mr bozo, shake his hand and thank him, with a wink. You have to play this off, no matter how you feel. She will either go on her way, slightly wiser, but her assesment of you fundamentally unchanged, or she will decide that she undervalued you. If this is the case, she will eventually call you. If she does, take her back if you want, but lay down the ground rules from the start., and know that she has demonstrated that she wants you to take a stronger hand in the relationship.

Best advice, though, is to dump her. Period. She is being deceptive and is probably having you lick up some other dudes jizz, which is fine if thats your thing.