One of my bestfriends dating my worst guy friend?

My best friend knows i hate this guy, Mitch. Mitch called my boyfriend names, called my other best friend names and called me names. He insulted my family to. My boyfriend would yell at him. Amy(my best friend) knows about all of this. But...she still texts him and talks to him. I've told her how i feel about it, and all she did was give me the silent treatment for a week. A few weeks after we didnt talk, she started telling everyone she had a new boyfriend. Well, being she used to be my best friend, i asked her who it is. She wouldn't tell me. So i shook it off and didnt really care from then on. Well i never even thought of who it could be. Yesterday, she brought me to Haunted Trails and said she wanted to talk things out with me. I went because i wasn't going to deny someone who wanted to be my friend. She told me Mitch was her boyfriend. It took everything i had not to scream at her, and tell her what a huge mistake shes making. I know i cant control someone else's feelings but shes supposed to be my best friend right? Girls before Guys? That's what she always said. "We'll never let a guy come between us." is what she said. But, apparently rules change for Mitch. Mitch used to have a huge crush on me. I never liked him. He called me "babe, baby, cutie, sweetheart," ect. And he used a bunch of cheesy pickup lines. Like when i told him i fell down the stairs he said "I wish i could have been there to catch you, babe" I mean seriously,? My boyfriend doesn't even call me babe yet. I know hes going to eventually break her heart. I cant stand watching this happen. Any suggestions? Thankyou if you do answer!!(: