i dont think its really what she needs to hear but the pre-communication beforehand between you and most importanly her and your potential escort.
I think its amazing that this is her idea and shes intrigued by it, usually its the man's. I would allow her to have first hand email or even phone communication with the escort. Where both women ar not the hired member the women need to digg each other to let another person into their intimate dealings most times. The more comfortable she is with the escort and the more honest she is about primary ground-rules (which are always a starter she should think about before diving head first into it, and if she gives the go-ahead can be broken in a session if shes comfortable) then you and she will have the best experience.
I tell many prospect couples I see that between them there is to be a very open communication of what they are comfortable seeing their SO doing to another person. Sometimes if people are not honest with their wishes it can end up being a very sticky situation. I have one couple I see where the woman doesn't like her SO kissing me passionately or even penetrating me. There is still a lot you can do in those situations tho! but its good to be honest and let her communicate privately with the escort as well so she feels like the choice is not made for her but as a compilation of both your comfort levels.
Good luck in your search, and remember, its not about what you say to her, its about what you do....its about a group effort that will, in the end, make everything go easier :-)