A close male friend of mine is having an affair, definitely emotional and probably physical. I don't really know his DP, only met her twice in about 4 years of knowing him but from bits and pieces he's told me, his marriage has been up and down for years, no DCs and they've come close to splitting up, only staying together for financial reasons at one point. Friend has been becoming closer and closer to OW for several months but continuously denies there is anything going on. However, from seeing them interact it's clear they are very, very 'close' and I'm sure he knows that I don't believe his denials.
My philosophy is you finish one relationship before you start another, or you have an open relationship which involves no keeping one person out of the loop. I don't know if the open relationship applies to friend's marriage but if not then my opinion of him has plummeted. But he is a genuinely nice person and if his marriage is a mess then I can see how this has happened and I want him to be happy. I'm guessing the MN mob will slate him for cheating and say I should keep out but I'm not really sure how to behave around him at the moment.