Is a will-drafting lawyer's file protected by privilege once the will's validity is challenged?

Not in the same way it was during the testator's lifetime. Ordinarily, a lawyer's file and communications with a client are protected by solicitor-client privilege, but Ontario courts have long recognized an important exception once that client has died and their will's validity is being challenged: the deceased's own testamentary intentions become the central issue, and the privilege that belonged to the testator is generally treated as no longer standing in the way of producing the drafting file to help the court determine what those intentions actually were.

In practice, this usually means the lawyer who prepared the will can be required to produce their file — including instructions taken from the testator, drafting notes, and correspondence about the will — to the parties and the court, rather than withholding it on privilege grounds. This exception applies specifically to the will and the circumstances of its preparation; it doesn't necessarily open up every other unrelated matter the lawyer may have handled for the same client. Because this exception is specific to testamentary disputes and has real limits, both the estate trustee and the drafting lawyer should get advice on exactly what must be disclosed once a challenge is underway.