With very rare exceptions, a lawyer must never disclose communications or instructions between the lawyer and the client without the express permission of the client, as such communications are protected by solicitor client privilege. The privilege continues after death and is assumed by the executor or administrator of the estate.
Lawyers are the only profession that cannot be compelled to breach this privilege either by court order or interrogation. Other professions and institutional records are routinely ordered by the court to be produced in litigation but it is relatively rare that a lawyers file will be compelled to be produced unless the privilege has been waived by conduct or consent has been provided.
The law relating to solicitor and client privilege is well known and set forward clearly in Smith v. Bell,  S.C.R. 664 at 671:
It is rather astounding that Mr. Schreiber should be subpoenaed to give evidence on behalf of the defendants as against his former clients and that he should produce his complete file including many memoranda and other material all of which were privileged as against the plaintiffs and whether the plaintiffs’ counsel objected or not that he should be permitted to so testify and so produce without the consent of the plaintiffs being requested and obtained.
Lord Chancellor Eldon said, in Beer v. Ward (1821), Jacob 77, 37 E.R. 779, at p. 80:
…it would be the duty of any Court to stop him if he was about to disclose confidential matters…the Court knows the privilege of the client, and it must be taken for granted that the attorney will act rightly, and claim that privilege; or that if he does not, the Court will make him claim it.
Because the solicitor owes to his former client a duty to claim the privilege when applicable, it is improper for him not to claim it without showing that it has been properly waived. Especially is this so when, as here, the circumstances are such as to make it most unlikely that a waiver would be given. Also, because it is improper to induce a breach of duty, I have serious doubts about the propriety of putting to a solicitor questions that involve the disclosure of confidential information without first bringing in evidence of a proper waiver. In any case, because the client’s privilege is a duty owed to the Court, no objection ought to be necessary and the evidence in violation of the privilege should not be received.