Assuming OP was right, I’d frame it more as his having an obligation to the husband as a human being making the choice correct rather than as a lack of obligation to the wife making the choice indifferent. We should be trying to make the world better, not carefully demarcating the bounds of social contracts so we can find out exactly where we’re allowed to do as much harm as we feel like.
But I also think SadArtemis is right that OP, to put it charitably, got ahead of himself
As someone else mentioned, it probably would have been worth giving her an opportunity to give her perspective before deciding if and how to tell the husband. Most likely the final result is just telling him, but there are more marginal possibilities that it is worth accounting for and taking an extra couple of days wouldn’t have harmed you or him (yours was already a dead relationship by virtue of your understandable disapproval of her cheating, so it’s not like you’d be enabling her to cheat for longer unless you think she’s managing to juggle even more guys along with young children and a marriage).