I'm very firmly in the "it depends" camp.
But my basic instinct is to keep the hell out of that subject.
I might override that, if I were convinced that the mate wasn't aware of it, and would be better off if he were. But .... different people live different lifestyles, and have different arrangements between partners. It is not beyond the bounds of possibility that mate already knows what his spouse is up to. Perhaps they have a bilateral agreement in place about away games. But it's equally possible that that is not something my mate wants to discuss with me! And may me very embarrassed, even angry, if I bring it up.
Then again, if she's playing away and he doesn't know, is he better off if I tell him? It may be a temporary thing, and about to end, and they'd subsequently have a happy married life. But if I tell him, it may well wreck any trust between them and possibly end the relationship. Is it my business to make that call and drop that bombshell on him? No, not really. Will he thank me for it, or is ignorance sometimes a blessing?
And what about kids? If there's any, and I blab something that ends the marriage, what about the impact on the kids? Do I want that on my conscience? Hell, no.
Ultimately, I would need a VERY convincing reason to get involved in what could end up a very messy situation, and in general, I'd keep quiet. Not happily so, but quiet nonetheless.
But my optimum situation is to not know in the first place. It's their business, not mine, and I REALLY don't want to know.