Conundrum?

So let's say you start at a new job, alright? You and one of your colleagues get along quite nicely. He introduces his new wife to you, and she's cute like a button... Your colleague seems to love her very much.
A couple of days later you see your colleague's wife making out with another guy.
So what do you do? Do you tell your new "friend" that his wife is cheating on him? And possibly destroy their relationship.
Or do you just pretend you didn't see anything since you think it's really none of your business?


I'm not really sure what I would do. Since I don't really know the guy that good yet, (the colleague) it might be better not to get too involved.
On the other hand, since my new colleague and I might be working together for a long time maybe I should tell him what I saw right away.

But I don't know. What would YOU do? And why?