Here's what I got out of it- and I may be entirely wrong- but Doms are all about control. Self-control is essential. So I don't think the mistake that was made was actually about the physical harm he did to her, but about the fact that he went further than he wanted to because he was not in control of himself.

However, if she was not damaged by this, I don't know why she would need help setting up boundaries, since they are obviously different than his. I would think a note of caution to her new Dom would work out just as well.