Might you consider setting a boundary on this NDY? Letting her know that you regret the recent discussion was seen by him and he was upset. Letting her know that you don't intend having any further such discussions if he is present. And if she begins one, calmly tell her - we can continue this discussion at another time when we are alone.
If she continues despite this - calmly remind her of the boundary and then walk away. It takes two to engage, and if you just remove yourself calmly, she'll have to argue with herself.
My own opinion is you can tell her this or not. However live it, this is a great boundary and if she spews at you, calmly walk into another room, without saying a word.
