FRIEND ISSUES! DX WHA SHOLD I DO!?

My friend Jack was texting my there friend Mary. And apparently Jack said something about my other friend, Ben. Mary told Ben what Jack said. Ben got really mad but Mary said that she defended him...But I HIGHLY drought she did! She calls Ben ugly behind his back all the freaking time! And I always tell her to stop! But, Ben believes her... and now every Time I'm with Ben and Mary, they completely ignore me! Ben thinks Mary's better friend than me! Its sooooo frustrating! She is suck a hoe to! She flirts with every guy she see's and when I tel her about it she says "is not flirting if you don't like them..." I can't stand it! she always gets the guys too I'm sooo..jealous... But I don't want to say anything about Ben and mary... I might lose a friend, or two. Should I just wait and let him see shes a bad friend? or what should I do?