You're in a sticky situation. You do have every right and reason to be anxious. She's done it before! She has the morals of an alley cat. I really encourage you to sit down and have a heart to heart with your fiancé. He needs to know what you are feeling and why. Why do you consider her a friend? She's a snake in the grass. Let your fiancé find another friend with more integrity and character or go out with them every single time. Otherwise there will be friction and anxiety. Good luck and best wishes.