Greetings,
Sorry to hear of your loss.
The IRS Form CP 575 is the actual letter you receive from the IRS granting your Employer Identification Number.
One situation that I am familiar with is that form is needed to apply to be a participating provider for Medicare. See PROVIDER ENROLLMENT
Any health care provider entering the Medicare program or any existing Medicare certified provider undergoing a Change of Ownership after July 1, 1997 must complete a HCFA 855 application.
Other agencies may also use the form to verify the correct EIN is being used.
The best way to find out why it was requested in your situation would be to ask the requesting person.
Best wishes.
Enrolled Agent
I have prepared individual, trust, partnership, and corporate taxes since 1987.