Do you have a problem with your wife doing her besty as well?
I would lay out ground rules so you know if there is anything you are not allowed to do, ask her if she would like to be part of it because it would be really hot for you to share her with her friend, and hen focus on your wife much more than her friend. If she just wants to watch you do her friend and neither of them wants it to go 3 way style, then ask her why, make sure it is really what she wants and she is not just asking you to because she thinks it is what you want. Assure her that she is the only girl for you but you will do this if it makes her happy or gets her going. If you make it all about your wife and her wishes, not about you at all, then she can't come to you later and say she wants to do your friend next. You should let her know that you wouldn't be comfortable with that now before an encounter with her friend.
There are a lot of reason to maintain monogamy in a relationship, but if both partners are open to it then it can be even more enjoyable to go outside the normal box. Jealousy is always a factor so you need to be sure that your focus is always in the right place, but sometimes bringing in a trusted friend is hotter than any toy you could ever buy - an experience you have both fantasized about and it will liven up your normal sex-lives as well.