I think she isn't 100% lesbian
My understanding is that even the most lesbian feminine woman or the gayest male can enjoy the company of the opposite gender. Very few of us are 100% gay or straight, even if we have never wanted to do things outside our self declared label.
Bi-sexuality is more common than one might think, even when one's relationships have always been straight, gay or lesbian, I know more than one lesbian who started her life being attracted to men, it was only when they reached their 40s did their preferences change. The NY Times, my arbiter of reality, had an article recently about this, though I also know lesbians who were attracted to women at an early age. We humans are a confusing bunch.
I have known guys who call themselves gay, some still appreciate women, but find it easier to declare themselves gay rather than bi-sexual, while nominally straight men will live in the closet.
Regardless of your husband's friend's stated preferences, they are having an EA, an emotional affair at the minimum, and he much prefers her company to yours.
I hope counselling can get the EA on the table and under mutual control, I understand your concerns. My wife has been rejecting me these last two years. No affairs on her part as best I can tell, she has increasing OCD or ADHD or depression, wrapped up in herself, rejecting my overtures, I hurt badly and recently decided I will leave this fall.
We are both in terrible places, I'm older than you, have been divorced once, w/o kids it will be difficult, but much easier than the first time - 1984.
I hope you see improvements, sounds as if he needs what the Car Talk brothers call a "dope slap" from your therapist. He is either blind or using the "lesbian" as a way to avoid you.