You cannot stay friends with someone who has threatened the marriage you want to keep. It's as simple as that.
To help you properly, can you tell us the timeline for this, like I suggested earlier? I'm pretty certain that they didn't suddenly develop a mutual attraction to eachother at his leaving do, so tell me this:
When did they first start fancying eachother and how long ago was this? A year ago, 2 years?
What happened after the kiss and when was this? That is, how long ago?
Why did they only meet twice, if this started in August?
What defined this relationship? It doesn't sound like it was the sex. Did he become addicted to the feelings the relationship was evoking or her as a person? They are very different things.
When did he start to detach from you and his marriage? Before he realised she fancied him/he fancied her or after?
You both sound rather ambivalent about this marriage and what has happened; a rather anti-climactic under-reaction, if anything. Now I understand that your communication has been poor in the past, but this is the chance to improve that - and yet there's no sense of urgency or clarity about how you both feel or what you both want.
Now maybe that's the case and it's not just you being guarded about what you reveal, but if you want to unravel what happened here, you need to know these things and I would be very happy to help you, if you need it. But at the moment, there's not enough in your posts to go on.
Incidentally, unless the work-based counselling specialises in infidelity, I don't think that's a good idea. If money is tight, you would be better off buying two copies of a book called Not Just Friends by Dr. Shirley Glass, which is IMO, the best affair recovery book on the market and has exercises within it, for a couple to attempt.