How to make him see?

my best friend of 12 years is an awesome guy named ron. his gf of almost 4 years, emily, is treating him soooo bad and he doesnt see it. emily and i are sort of friends too and she will call me to talk and tell me about how she flirted with this guy, kissed this guy and so on. i want so bad to sit ron down and let him know what shes doing to him but i dont want to seem like a 'tattle-tale' or i dont want it to seem like im trying to cause drama and break them up. our inner circle of friends all agree shes not good for him and hes really going to get hurt, but he wont listen to us. emily is his first and only gf he's had, shes cheated on him twice and he took her back, and then she tells me all this stuff she doing, trusting i wont tell him. first i dont get why she would tell me when she knows hes my best friend and i want to tell him, but i dont want to lose her trust either. what do i do? =(