How to tell your friend you slept with her boyfriend?

OK SOOO... before i was friends with my friend(judy), i slept with her boyfriend(bob) while they were on a break. after i slept with him, i became close friends with judy. bob and judy are thinking of getting back together, but bob still keeps trying to talk to me behind judys back. so i want to tell her what happened between me and bob because i feel like she deserves to know who he really is before she gets back together with him. he has also hurt her countless other times but she always takes him back. my other friends and i think that she should be done with him and that telling her what i did will get her to wake up from this fairytale and move on. the problem is, i dont know when where and how i should bring it up to her :(