back stabbing bitch!!, what should i do? Options

[jEllyBeaNs]

ok im not sure where to start...but my best friend,lets call her Sam, well sam i USE to call her my best friend all until she became a back stabbing bitch! Sam is the type of girl that thinks she can get any guy tht she wants pretty much and since our freshman year in high school she has gone out with or tried to talk to all my ex's that i had broken up with. so during my most recent relationship i told sam that im really in love with, (lets call him bob) bob and tht he's nothing like the other guys i've ben with and i told her that she would really like him. But through out that time when i told her about Bob she never tried to talk to him. so about 3weeks ago wen myself and bob decided to break up (because of things with my mom) all of a sudden bob tells me that sam has been talking to him on myspace and that he thinks she likes him. i was heated!! i told him that i wasnt happy and that i knew what she was tryin to do. i told him that she is going to try to get with you and that i dont want him talkin to her. bob then told me that he's not mean and that he's not gonna stop talkin to her cuz he jus met her...so i told him what all she has done to me with my past relationships and he promised me that he would stop talkin to her and that he would tell me what she says to him on myspace. but the thing is that i trust him but everytime i think about the situation i get really upset and angry and i dont kno if i should talk to her about it or jus let it go. its really really bothering me and i dont want to bring it up to bob again because i dont want us to argue about it again. i need some help