Probably pretty easy to show she is unbalanced. And I'm guessing others have experienced her Karen attacks as well and would probably also chime in.
I agree good on him for not outing her...but if she were to follow through with her threats then he absolutely should defend himself on her posts.
I guess the problem is the originator of posts can delete responses to their post. So he'd probably have to share her post out with his responses on it as well.