Anyone have advice on a threesome?

My wife and I are thinking of bringing her friend into the equation for her to be able to experiment with a girl. We are both open to the idea and the thought is entertaining. Her friend has been with girls before and would make her the ideal candidate as she has agereed she would like to have fun with my wife. Here's my dilemma, my wife gets in the moment and likes kinky things during sex. When her hormones are highest she generally thinks more of how much she enjoys taboo. If she and her friend happen to follow through and it is agreed upon that it's for my viewing pleasure and I'm only to have involvement with my wife, if she tells me she wants to see me have sex with her friend or do anything at all with her, do I go with what she says or should I stick to the agreement seeing as how decisions during sex can be look at as repulsive afterwards? Yeah it's about meeting her satisfaction, but ultimately it's the strength of the relationship that matters most. We have already started small by role playing the situation where we each pretend to be someone else and her pretending I'm a girl going down on her and are both pleased with the outcome. I'm not looking to get in a night with her friend, it's all about my wife getting to experience another part of her wild side and it being together.