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

The deal was probably that the acccountant wouldn't be charged for helping evading taxes.