I'm going to tell a person a pretty bad secret about my friend
Why do you want to spread secrets about your friend?
This friend didn't keep my secret to herself, she shared it with someone.
While it is understandable that you are upset at your friend for sharing your own secret, it isn't right to go around and do the same thing to your friend as an act of revenge. Two wrongs do not make a right and secrets should always be kept private when told in confidence. Instead of trying to retaliate on your friend, try to talk with her about how hurt you were over her actions and give her an opportunity to apologize and make up for what happened.
Okay I'll talk to her about it.
It's good you want to talk to her about it. I'm sure she'll regret about what she did to you. I also hope you changed your mind about sharing her secret with people.