I ran into an old friend who asked me to keep a secret?

This old friend is a whore. She will sleep with anyone. She has slept with a boyfriend of mine and my sisters. I talked to her to get info about something but she confided in me. She told me she is sleeping with the husband of a great woman I know. Now the couple where high school sweethearts and have been married for about three years. He stays at hotels overnight with her. She hugs his wife and kid when she see's them. She made me swear I wouldn't tell. What do I do with the info? Should I let it play all out? I having to see things go so wrong and know I can change things. This girl is mean and crazy enough to kill.