I'm not in a situation like this. However I imagine it comes down to how long you have known the friend, if you know or like their mate at all, are they "dating", living together, or married. For the most part men usually stay in their own lane. If it bothered us we'd just tell them to not talk about it around us. I highly doubt any guy has dumped a good friend because he was cheating on his mate. That's pretty much considered his personal business.