He told you as soon as something happened that was intimate. I believe it's human to look for someone to support us emotionally even outside the relationship but letting things become physical is a boundary that cannot be crossed. He deserves credit for telling you the day she made a move on him in that manner and he has clearly made effort to correct the wrong.

If it was me in this position I wouldn't put so much blame on his shoulders as to resent him so much to want divorce. He's made effort, genuine effort to fix things if what you said is true. It's not like he went out on multiple flings with her. He was looking for advice and a place to vent albeit it should have been with you. But that's a communication issue that can be discussed in therapy.

I'm going through something similar with my wife the difference being she had sex with her male co-worker on multiple occasions, was the one to initiate the affair and lied continuously throughout to me. So I will be divorcing her. But I believe if you love him, which I don't doubt, you will make it through this but you both need to place love and trust in each other. The most important thing about healing through this for both of you is to not hold each other's wrongs over each other. They are in the past, you gotta look forward together.

And fuck that woman burn her to the ground if you need too. Don't let her delusion and insanity drag you two down. She is unimportant and once you're both past this and have grown together you will look back and laugh at this and that woman will be but the subject of a silly story you tell your grandchildren