> When they die, it becomes part of their estate and it's not confidential anymore.

IANAL, but I don't think the law works this way. If you reveal any of that confidential information, you would be as liable to getting sued for breach of confidentiality as your father was. I believe this will be one of those cases where ignorance of the law will not protect you, beause it is common knowledge that what happened between your dad and his clients is confidential information.