I slept with my best friends boyfriend?

It was a long time ago before we were friends, she was just a girl that lives in my building for years. We were friends but not close just thrown together thru mutual friends. Once i was having prob with my bf and they were broken up, he was there 1 thing led to another. Something happened to a mutual friend that kind of brought us closer 2gether and we've been best friends since then. They got back 2gether. He promised neva to tell and we both kept it a secret. Dont know why but she called this morning crying that he told her everything and she'll neva. I told her ill come see her when she gets home. I dont knw wat to say to her i feel terrible