A few thoughts from me on this one...
1) She is creating a hostile workplace for you and co-workers. The big boss is aware of this and should have done something about it, but big-boss didn't. So now she has nothing holding her back. However, it could become such a problem with other co-workers that they eventually sue the company.
2) Ripping her supervisor (a new one), is insubordination. This is not a social club where everyone is on equal footing. She reports to you. You are her manager. UNACCEPTABLE. It's also a cause for termination in every company I've worked for.
3) If she loses her job over this, it is NOT YOUR FAULT. It is the natural consequences of HER behavior...behavior SHE CHOSE to engage in. Not once, but several times.
Report her, have her written up and if it happens one more time...sounds like time to fire her to me. Letting one person control the entire company through their bad behavior makes the rest of the employees feel they are not valued and you guys could end up losing a lot of really good people.