Help! :(

I told one of my best Friends that our other friend was gonna b asked out sejak the boy she likes. (i asked him before i told my best friend but they are close Friends too) As soon as i leave she tells our friend and the guys she likes likes her. So now its not going too b a surprise. Luckly my friend told me my best friend told her and this isnt the first time she has done something like this. I cant tell her i know because i promised my friend. What should i do?!?