Few insignificant details changed below so people involved can't be identified...
DM called this evening and told me all about her first date with a man she met online, we'll call him Paul for the purpose of this thread.
Midway through the conversation she said "oh you might know his kids, they went to your school, I think he said their names are...".
I do know his kids, they were all lovely and though we were friendly and we have each other on Facebook, we're not in touch and not likely to ever be other than the odd congratulations comment on major life events.
DM said she might go out with him again but not sure as something was "off about him" but put it down to first date nerves.
Because I'm a nosy bitch curious, I later stalked checked Paul's DSs Facebook profile to see if I could see a picture of him.
I only bloody came across Pauls wedding pictures from late last year 
I obviously called DM straight back and said DO NOT go out with him again, he's married. Apparently he told DM he wouldn't be able to bring her home for a while because he was living in his DSs small flat. Obviously it's actually because he lives with his poor wife. He also went a bit cold after DM said I'd probably know his kids.
Also a few other alarm bells but this post is an essay as it is!
DM shocked and annoyed but more upset for his new wife, if he's happy to cheat just a few short months after the wedding, he's probably been doing it all along. DMs not going to see or speak to him again.
Now, what do I do? Do I message his DS? If we were close I'd say something, but as we haven't said more than "congrats" in nearly 10 years I'm not sure I should get involved at all.
On the other hand, he's a prick it's awful that he's got no problem cheating on his wife and I really feel for her.
If it were my DP, I'd want to know.