Maybe I didn't do a good job of relaying the blow by blow of what happened. I did agree and repeatedly empathize with him that he didn't deserve the tone that she took with him, but it's not as though she cornered him at random to lay into him. The conversation started out fairly casual and calm on both sides, but rapidly escalated because he was being extremely dismissive, and the more dismissive he got, the more she doubled-down on what she was saying. She didn't really pop off on him until he said "I totally tuned you out," and by that point they'd been going back and forth for a while.

I think they both could have handled the situation better, but I was surprised that his takeaway was "Sam is an asshole." It was pretty obvious to me that they were both being unfair with each other - he was also being an asshole, just a quieter/less visibly agitated asshole.

I also acknowledged to him that I might be over-empathizing with Sam because this is a pattern that has shown up between us before - where I'm expressing something serious, he's dismissive, I get upset, and he gets even more defensive/dismissive because I'm upset. He has a habit of dismissing legitimate emotions in a really hurtful way when he feels threatened and it ends up escalating situations like this. It's something we're actively talking about and dealing with, but I don't know if he realizes how it shows up in his other relationships, too.

I do agree that they're separate issues, though. I said something similar about him managing his own relationship with Sam since it's really not my business, but that given how close they are and how much love there is between them, it's in their best interest to find a better way to communicate about these issues. We talked once about whether or not he should get a dog and I laid out all the concerns I had and told him that it was ultimately his choice; we haven't talked about it that directly since. But honestly, if he was actually going to go through with it, I feel like I would want to have another conversation strongly urging him not to, because it would be a really, really bad idea.