what do you do when you kiss your bestfriends boyfriend

well what happen was i didn't know that they were going out till that wenesday. right then and there i want to tell her but i promised the boy i wouldn't tell anyone. but i don't know if i'm a good friend by not telling her but also i'm a good friend for keeping my promise.but now i feel really bad . but also he said he was about to have a girlfriend so i don't know what to do? please help me with anwers.