I know how this feels. All I can say is talk to her about it. If it turns into a big fiasco, just remember that shit happens and he is just another boy.
Talk to her about it. She may have been scared of your reaction. Sort it out with her first, don't resort to yelling right away. It'll only complicate things further
well thats happend to me before talk to her about it if she did it on purpose and knew what she was doing then she is a backstabber if she liked him to and didnt want to tell you because she was scared how you would react then i wouldnt really blame her hope my advice worked