That is the thing, just because you can tell the truth, it doesn't mean you have to tell every sordid detail, especially when it is designed to hurt, and this particular truth is. It could have been left at Mom and Dad are not getting along, but this particular truth paints someone in a bad light, coincidentally your husband's ex. when I say it doesn't happen in a vacuum, that only means there is not one defineable issue that broke this marriage. It may have been the truth of Dad doesn't spend enough time with mom, so she had to find a good man who would. But when you cherry pick one event and blame the whole thing on her, he walks away smellinng so much cleaner than he is. I am not condoning adultery, never have never will, but I will say a little compassion doesn't mean a lie, and omission for protection doesn't mean your lying either. Would you hate for your child to be hearing negative things about you, even if they are true, and especially from the other parent. Yes, you would judge, nothing wrong with that, but you do not have to pronounce that judgement to a child. All the truth telling in the world is not going to stop the whispering. Trust me, you can show your children right from wrong without using the other parent as an example. I see this as the magnifying glass of divorce. Little things become big things, and we have to let people know the big things.
Baring her sins to her son, is not the best way to go. What she did was wrong. But she is not example you HAVE to use to teach right from wrong, and to teach morals.
A divorce is between a husband and wife, leave it that way. When it is said and done, usually there will be some pride swallowing in order to co parent in a good way. It is not easy, but the best interests of the child should count for something.