I've done both. Every other time, I stayed out of it and just minded my business. I'd see the woman lighting up my buddy over some random crap, and I'd just bite my tongue.
When it happened to my little brother, I stepped in and told him what I thought, even though I knew better. I just felt like I had to protect him. Long story short: I regret it.
Even with my brother, it was none of my business. Now, he and I barely speak anymore.
Of course he chose his wife over me. He will until/if they divorce, but by then it'll be too late. There will be too many miles and years between us to bridge the gap.
Misplayed by me. Can't take it back. I'll have to live with that. In retrospect, it was a very selfish move on my part.