Have you considered whistleblowing?
I'd called out fraud (blatant lying in investor updates) at a VC backed startup where I was a technical co-founder, once. I emailed all the investors and presented all the evidence to them. They decided to not rock the boat and keep my charlatan co-founder. So, I left. Now, the company is slowly bleeding to death.
Yes. But I'm not working at either company and I'm 99.9% sure that it would lead to absolutely nothing other than a lot of misery for myself. The NDA's I sign have some pretty stiff penalties attached. I was actually hoping to see my trust in the auditing company confirmed and I'm still more than a little bit annoyed that they did not respond in a more constructive way.
My response however is a simple one: I used to steer (a lot of) business their way and I have stopped doing that.