It is, as you mentioned, one if not the only time, we get both POV's and from the women involved. I too thought Sophie had whipped Diana's face when I'd first read it. It was not until Diana mentioned Sophie got her attention by threatening to whip her face.

I am still not sure what provoked Diana to begin so with Sophie about Jack other than to get Sophie to make a decision about whom she loved, Jack or that Mr. Bowles creature.