Honesty , is seriously always the best policy. Everything gets out eventually and you'll be the one who wants to tell her. I doubt she'll be mad if it was a amicable break up.
Thanks for responding....we started dating after they broke up. They broke up in late November, and haven't been together since. But she has liked him for a while after that so that's why i'm holding back. And also I dated him for like two weeks in January but we broke up because of this friend so i don't want to ruin it again