One of my closest male friends is in a relationship with a woman who I now know to be cheating on him. I found out because I have a friend who is friends with the girlfriend (do keep up!), who told me what was going on, not realising that I am friends with the male half of the couple. I have met his girlfriend a couple of times, but I do not know her well so it's easy for my friend to make the mistake of thinking I didn't know the couple she was referring to.
The girlfriend is cheating with an ex. They all used to work together, but the ex has now left and works somewhere else. I don't know how they got back in touch. I have been informed they are sleeping together regularly but not overly frequently (not more than once or twice a week at the most) and it has been going on for about a month. I don't know whether this is just a fling or something more. As soon as I realised who the couple in question were, I shut the conversation down and changed the subject, as it felt wrong. The friend I was talking to still does not know I have made the connection as to who it was.
So if you were me, would you keep quiet about it? Or should I gently tell my friend?
Sorry for this being longwinded, I wanted to give as much information as possible to gather opinions.