She's easy to talk to, they have similar interests, she understands him etc etc.
But then started saying he can't talk to me, he's lonely, I don't make him feel special. I had to walk away at that point.
Bastard.
Negging, self-justifying BASTARD.
To his wife who loyally stuck by her impotent husband.
Bastard.
You know he's trying to make you do the Pick-Me Dance, right?
www.chumplady.com/2012/04/the-humiliating-dance-of-pick-me/
Have a browse through her archives - she;s a wonder of wisdom, solid advice, & snark, she;ll do you the power of good.
It's just the fact that there have been problems in our marriage which we have been working on. We were in counselling last summer. Now I find out that he's been discussing all of that with her.
😡
That's as big, if not a bigger, betrayal than if he'd shagged her.
I am SO sorry OP 
The bigging her up then putting me down doesn't sit well either. I've been on mumsnet long enough to know 'the script' .
Good. Forewarned is forearmed.
What keeps going round in my head is that after telling me how young, caring, interesting etc she is he then went on to say he was bored and I didn't make him feel special
See Pick-Me Dance & Chump Lady.
How special has this twat made YOU feel lately?
Does he even bother with you physically, or are you expected to be a nun due to his impotence?
Does he ask you about your day, cheerfully do his half of the chores without instruction, bring you tea in bed, hug you?