How would Mathilde Loisel break the news to her husband about the necklace being fake?

A fascinating question. On a simple, factual level, of course, we don't know. We have to extrapolate, and to take an educated guess about how much she has grown through the story.

I would say that her first response would be like the narrative voice's reflection: she'd reflect on how different things would have been. I'd even say she would break down in tears over having lost so much. Then, I'd say she would grow quiet and realize how much she had learned (I hope).