Issues with my mans friends!!

I have been dating my man for a while now and we are very in love. He is an amazing human being and everything has been wonderful. His best friend is engaged to a chick that I CANNOT stand. They have been friends for a long time (and trust I am not that girl who gets jealous of friends, even of the other sex) I am confident in who I am and confident in our relationship. The incident has happened twice now, to where we are all hanging out at a party/bar/etc. (night out with friends) and I will be in another room and she takes him aside and precedes to decimate my character to him. Says all kinds of things about me. I have hung out with this woman twice. So she clearly knows nothing about me. I have continued to take the high road and be nice and civil and respectable to her because I choose/want to be the bigger person. As well, it is important to my man that we be friends- what’s important to him is now important to me. But he does know how I feel. I have confronted her in a very respectful and mature way, but still seems to be happening. How do I proceed? My man understands my feelings and also knows how she is, but he respects how I feel and also supports me. But I can see and feel it’s hard for him because it’s his BEST friends fiancé. How do I proceed through this? For the benefit of both of us? I don’t do two faced people- I will always be true to myself and stand up for me (and he has as well) but I know how much it means to him. Basically I have to somehow make this work and need some serious input.