Answer:
Even though a client cannot force CPA to engage in illegal activities, like any other employer cannot force an employee to commit a crime, there exists a -accountant-client privilege that cannot be violated. This means that the accountant cannot disclose any information regarding past clients unless ordered by a court or authorized by the client himself/herself.
So, before discussing any previous events, the successor CPA must obtain the client's permission to discuss any events related to the previous relationship.