My opinion on this is:
In this specific scenario, what he doesnt know...wont hurt him.
Tell her to keep this secret with her to her grave, never say a word about this to anyone and spend the rest of her life being the best partner a man could ask for.
:)
And this is the only advice which is given by desi relationship experts all the time. I agree with it 100%. They even go to the extent that later on if the guy wants to blackmail her or her husband finds out some other way she should outright deny it. But before anybody does that she should make sure that her ex does not have any kind of proof of his misdeeds and her mistakes. If he has no proof then truth in this case is not going to help anybody. And if the ex has proof then the truth or lie will not save the marriage only the fate of the girl will save it.