For me, I always believe in treating others the way you want to be treated. So personally, if I was the betrayed wife, I would want to know simply coz I have a right to know. I mean, why hide the cheating from me, right?
And if I knew my good friend's HB/ wife is having an affair, then I think I'd know them well enough to know whether they'd want to know, or whether they'd want me to keep quiet. So, I'll act accordingly lor. But I doubt I can stay completely quiet also, haha, so if the latter, then I'd drop hints.