Should I tell her fiance that she's been cheating on him with me?

Sorry to say but I feel like she's just not good person if she could cheat on him don't you think she would do the same to you and poor guy ( her fianc�) he's supposed to be on a church mission . Honey there are plenty of nice girls out there and you should never lose your friends for nobody . I think you should just move on and I don't know if you should tell him cus you knew she had a bf and it would only be doing that to spite her . I wish you the best and hope you van find a girl that's just for you