The withholding should remain assigned to the return whose identifying number is listed. It's difficult for the IRS to adjust or change the withholding on a tax return and is not a good method. If there is excess it will be refunded on the return of the deceased (Form 1040). Filing Form 1310 will allow you to receive the refund in your name or the name of the estate depending on your information provided.
We are sorry for your loss.