Oh, how I love your input, Oz.
She has agreed to him watching. However, I get the impression that her interests in the situation are solely based on a "connection" between her and I. So, if halfway through the session, my Master decides that he wants to step in, I feel it might ruin her experience. Based off what she has told me, her BDSM sessions have involved bondage and rape scene scenarios. Yes, painful pleasure would be a factor, but I think I would take it to a higher level for her. Also, its not that I wouldn't want to share her... its him that I wouldn't want to share. The idea of watching my Master... my Dom... my Love... dominate another female hurts. I would greatly struggle with that.
If I did Domme her, I would be independent. There would be no teaming up on her. The only thing that has been mentioned is that after I am done with her, he would want her to sit back and watch while he Doms me. This actually comes off as a strange request simply because he has stated so many times that he doesn't want to share me... even if it was just being watched. I'm thinking that my friendship with her, with the level that its ay, is what has made him bend his own rule. He is quite aware of how much she means to me. As far as a boy toy goes... it is an absolute no. On both our parts. I am his and he is mine. With my Domme fantasy, he is supportive on a one time only level.
Both you and denuseri have pointed out relationship change. I will be the first to admit that I fear this. I don't want to lose a friend and above all... I will not be ok with tension between my Master and I. He means far more to me than any fantasy or sexual desire.
In my head I see the session being something great. I feel that her and I would grow very close from it both mentally and physically. I really can't find any negatives on it. Just a fear of altering relationships to a point where they fall apart. Being as its consensual in every aspect, I hardly believe that anything will change too dramatically. I will understand Domination better at the end and have a greater respect for it.