This has been the first of these so far that I can answer
I would not say anything. For all I know my mate knows their partner is cheating but is just pretending everything is fine, and knowing someone else knows might put pressure on them to end it when they dont want to. Hey, they might even be HAPPY with the situation as it is...blissful ignorance. To me, what goes on behind closed doors is none of anyones business...and its never a good thing to get involved in other peoples relationships as usually you somehow end up being the bad guy
But anyway, no way would I risk losing my job for something as silly as this