I’m sure they were able to put pressure on her in order to stay silent; or, they had a gag order placed on her.
If she was doing it for him, she was likely doing it for others and thus it was in her best interests to stay silent for one to protect the revenue stream from the others.
The deal was probably that the acccountant wouldn't be charged for helping evading taxes.
Yeah, I feel like if I were an accountant who wanted to be able to keep practicing in a state without legal issues, and the state told me "we're pretty sure your client was doing something illegal and think you might have known about it, so tell us everything you know and don't tell your client before we arrest them and we'll take into account your cooperation", I probably would also do what they suggested