Alright, keeping in mind that if ones spouse gravitates to one of the "friends" I think there was probably too much wandering curiosity to begin with in either the friend, the spouse, or both.
Second, I think that once a serious relationship has existed that should really close off thoughts of your spouses friends, period. There should be equal restraint from the friends themselves. This is all from an ethical standpoint, of course, and I think what you witnessed is proof that "friends" might not be such at all but people who hang around for a benefit.
But, just to counterpoint, I think it is noteworthy to point out that love is love and truly cannot be helped. For example, I don't like the idea of interracial relationships but if two people are in love I think that is the bottom line and should be respected.