There is really nothing to do but bring it up to your wife (and be sure to do that before saying anything to your friend!). It would be pretty unrealistic of her to think that close friendship and ongoing sex couldn't possibly lead to feelings, so hopefully she will understand, and you can always choose to be clear that it's just a feeling at this point and that you won't make a single move without her go ahead. Then you two can decide together whether to say anything to your friend.
If you can't think of how to begin to talk to her about it, try doing some reading at www.morethantwo.com
, which is a site that I and many others have found helpful. You can steal any of the language that resonates, or just direct her to any parts you like.
If your wife *is* understanding (here's hoping!), please try to avoid the common and potentially fatal mistake of thinking that since both of you have been involved with your friend sexually, both of you should be involved with her romantically too. If your wife has been harboring the same feelings that's one thing, but if not then trying to make this a romantic triad (all three moving forward together) versus a vee (one person having a relationship with two people... as in my sig line if I was your friend, you were Gia, and your wife were Eric) will likely create problems and drama. Actually, let me recommend this article in particular: http://www.morethantwo.com/coupledating.html