Your first mistake was to assume that an IRS employee knows how to handle this. It’s probably the first time they have come across the situation. Your job is to tell them what to do, and you can find that out by researching the online Internal Revenue Manual that they are supposed to be following. It’s really not that difficult, I just used “Form 56" for a search since you got that far, but “decedent disclosure” probably would have worked also.

So this is what you would have found:

11.3.2.4.11 (09-17-2020)
Deceased Individuals
The administrator, executor, or trustee of an estate may receive the returns and return information of the deceased individual. Often, an estate is already settled at the time a request for returns and return information is made, and thus an administrator, executor or trustee of the estate will no longer exist. In these circumstances, a person seeking disclosure will often need to consult state law to determine how appointment of an administrator, executor or trustee of the estate may be accomplished.
Any heir at law, next of kin, beneficiary under the will, or a donee (recipient) of property may receive the returns and return information of a deceased individual. Such person must have a material interest that will be affected by the requested information. A material interest is an important interest and is generally, but not always, financial in nature.
The requester must furnish satisfactory evidence that he/she is an administrator, executor, trustee, heir at law, next of kin, beneficiary under the decedent's will, or a donee (recipient) of property. Acceptable documentation includes but is not limited to: birth and/or death certificates, letters testamentary, a will, or other court document.
Note: For deceased taxpayers, the person whose name is shown on the second name line on the IDRS Entity screen can be given information if they have furnished satisfactory evidence they are an administrator, executor, trustee, etc. The Form 56, Notice Concerning Fiduciary Relationship, may also be considered when reviewing for satisfactory evidence. See IRM 11.3.2.4.8.
If the requester is an heir at law, next of kin, beneficiary, or donee, he/she must show a material interest which will be affected by the requested information. In some states, however, simply establishing a material interest may not be adequate.
Note: IRS Employees should check with local Area Counsel to determine implications of state law as it pertains to material interest.

11.3.2.4.8 leads you to 3.13.2.19.1, which has this useful information:

When a paper Form 56 or correspondence is received, the following information is needed to process the form:
Taxpayer name line information (Part I - Identification),
Employer Identification Number/Social Security Number (Part I - Identification) (if missing, research IDRS),
Fiduciary name and address information, (Part I - Identification),
Authority, (Part I Section A) One box in Item 1 must be checked, Item 2 must be completed if applicable,
Tax forms to be filed, (Part I Section B),
Court and Administrative Proceedings, if any took place (Part III), and
Signature of the fiduciary (Part IV).
Exception: There are situations where letters testamentary and/or court certifications may not be required. When a decedent has passed without a will and there has been a transfer of their estate to a surviving spouse or other heir under the property or estate and trust laws of the decedent’s state of domicile, courts may not be involved. In such instances, the Service may accept a signed Form 56 without additional supporting evidence.