I think you both should go. It's his best friend. If the fiancée decides to create a scene, it will be pretty obvious that she's instigating trouble and then Z will understand the next time when you have to decline an invitation to be near her.
If you don't go, you should be truthful with Z in a respectful way without trashing his girl. Your boyfriend should say something like: "we would both really like to be there, but after the recent drama over texting, it doesn't seem like a good idea. We're both really uncomfortable with the amount of conflict."
Thanks for the quick reply. Actually, my BF has already told Z that we are uncomfortable with the amount of conflict- but Z persists. She's created drama before ( not with me, but my BF over other things)
The other problem is - Unfortunately, I am seen by all the guys in the group as the "hot one" and I only know this because she herself told me that when we first met. I thought it was weird that she said that to me, and now its just turned into jealousy because Z visits us almost every other day, and she's finding excuses to keep him away. It sucks because I love my man ( we have marriage plans) and have eyes only for him. Its VERY uncomfortable. My BF has been tempted to lash out at her but I keep holding him back out of respect for Z.
In my head - I REALLY would prefer not to go, but I do see yor point - if she creates a scene, maybe he'll see her true colors first hand.
I definitely understand you. I would never tell him he can't go to the party himself (and I know you are a reasonable person too). But if you don't want to attend, I don't think anyone could really blame you.
This drama is not your fault and if Z is unable to stand up for himself and his friendships, then unfortunately, he is going to lose his friendships. It is sad, but if this crazy girl has decided to make life hell for you, then there isn't anything you can do except avoid her.