Ah, the part about becoming a stay-at-home mom with a 1-year-old changes things in my imagination all right. Major life changes for both of you, but especially for your wife. Is she feeling off-balance and uncertain about her life, do you think? It sounds like the counselor suggested that, in a way.
I dunno if the counselor covered primary and secondary emotions...? A lot of times emotions like anger and resentment are the conscious manifestations of deeper feelings. For example, when my son was involved with meth addiction I spent days in a blind rage, but the real emotion underneath was fear. I couldn't understand and deal with the rage -- it kept coming back no matter what I could do -- until I recognized and dealt with the fear that was deeper down.
It might be useful at some point to probe behind the feelings that surfaced when you discovered your wife's friendship. Maybe not right now. Sometime.
A personal perspective: I'm "dating" a woman who's in a poly marriage, and my understanding of her husband is that he is at ease sharing her affection because he has become absolutely confident that the two of them have an indestructible love for each other. And he's right: she connects well with me emotionally and intellectually as well as physically, but her connection to him is a thousand times deeper.
Anyway. Maybe at some point you will see something down there in your psyche, understand it, and realize that your relationship with your wife has become so strong that you can let go of whatever it was and not worry about her friendships. Because you will then know that no outside friendship can ever threaten what you and she have...and at that point she may be able to tell you everything about her friends with no sense of worry or guilt.
Or maybe I'm just playing amateur shrink here?
Take everything with a grain of salt. You're there and we ain't.