Hiring her was a lapse of judgment. The rest of it sounds like a good time. BDSM relationships involving power exchange can be healthy but there is a huge risk that a messy breakup can go this way.
If my wife and I ever got divorced, I know she'd have the power to rake me over the coals with receipts. So I can give the guy the benefit of doubt, because based on what is alleged and my own personal experience it sounds reasonable that it might've been completely consensual at the time.
However as we grow as people, we can recontextualize our experiences and decide that hey this was really unhealthy and he should've known it was unhealthy and that she wasn't capable of consenting, and that could even be right. Some people give enthusiastic consent and it turns out to be some PTSD trauma response. Given the number of people in kink with trauma in their past, the lines can get really blurry.
I'm not saying she wasn't abused for sure, just that from a kink perspective his side of the story seems as plausible as hers. Regardless, I hope justice, whatever that may be, somehow prevails. But this case is going to hinge on whoever is more credible on the witness stand I think, and less on indisputable truths.