tell your friend not to tell you about the someone that talked about you. tell her whenever she hears someone talking about you, to not tell you because if she comes and tells you about her, maybe a few months down the road, she is going to be the person talking about you.(also tell her if she were a real friend she would stop that person thats going to talk about you before anyone hears what that girl has to say.)

Work out exactly how important this issue is. Is it worth getting your friend into trouble? Why not just blank this girl from now on, don't worry, she'll know exactly what she's done. What do you hope to achieve by confronting this girl? A slanging match will make you both look bad. Much better to take the high road, it's much cooler.

Just let her know your pissed with her but don't let on why, but really it does depend what she said, most girls say things they don't mean when they're annoyed, is it really worth causing a rift with your friends over?