I've know this man about 15 years, we were colleagues initially, but haven't been for a long time. We still work in the same city and meet for a drink after work maybe 5-6 times a year. A very occasional text in between, maybe a joke or something about the football, but nothing else.
That said, I do count him as a good close friend. When we are out together we generally spend the first hour talking about work/children, then move on to football and once the beer is properly flowing will set about putting the world to rights. There's not much I couldn't talk to him about and we always have a good laugh. We are both football fans and TBH I don't know many people I can share this interest with - DH not interested and neither are my female friends. He has two DSs 7 years older than my 2 and he coached his DS1's football team from when they were 5-18, so he has become my go to for advice on raising boys. He's always treated me with complete respect and I have no reason to suspect the friendship is anything but platonic on either side.
However, something he said when we were arranging our next night out made me think he's not intending to tell his wife who he'll be with. I think because he likes a quiet life and it's easier not to than because there's anything to hide, but even so....I always tell DH exactly where I'm going and who with and if he's awake when I get in, will relate much of our drunken conversation - although I suspect he doesn't listen
There was one other occasion when I suspected this, but when I challenged him, he just said "she doesn't ask" the evening progressed and I didn't think about it any more.
I've never met his DW, not deliberately, I don't think, we live in opposite directions 40 miles out of the city and have never socialised other than after work. He doesn't talk about her much, but when he does, he has occasionally had a gripe about how her (freelance) work took too much time away from DC, especially when she was making no money and he was working all hours to support them, but he was also incredibly proud when she sold her first work.
So, anyway, what would you think and what, if anything, would you do? I am uncomfortable with it, but he is an important friend to me. I don't think issuing an ultimatum would help, he'd react badly to being pushed into a corner over anything. I also wouldn't expect to be able to tell any of my other friends how they should deal with issues in their marriages