What To Do If And When Your Friend Is After Your Fiance?

Please, fellow nairalanders. What am i supposed to do? A friend of mine(i suppose i shouldn't call her that anymore) is after my fiance. I've got this very wonderful relationship with this very wonderful man who i'm so in love with and it happens to be the same thing the other way round. We share all confidences and recently i found out that my friend who is also his neighbour has been trying to seduce him by throwing herself at him. She wears skimpy things around him. She's been telling him things i had shared with her as woman to woman just to discredit me. She even went as far as telling him that we weren't married as so he's still single when he reminded her that we were engaged and she's supposed to be my friend. Please, i want honest and sincere advice.