I feel you have encountered an issue that is common when you use friends and that is the necessary communication is not there. Let me try to explain as friends you have a style of communication and based on your history together you can understand each others needs. Then when you expand your friendship by introducing the idea of a foursome there is an underlying assumption that there is not a need to talk about the particulars of the threesome. Since you do not talk about the particulars of the threesome boundaries get crossed, feelings get hurt, and the friendship over time becomes strained.

Reading your posting it reads as though there are some issues that talking about it may not help. The three I am left to question if they exist includes is there feelings / attraction between your wife and your friend. Second question is the other wife having problems with jealousy. Third is there an underlying relationship issue between the two of them. If I were to offer advice on this situation and provided your wife is agreeable to it I would recommned finding another couple. From my experience I would have to say this is not a good situation and no amount of talking will change it. It is best to work at maintaining this friendship and finding another couple.