I am not clear on if she passed this year or a previous year so I am not sure if you are doing a final personal or estate return so I will cover both.
If you are filing a final personal tax return, then the ID would be her social security number.
If you are filing an estate tax return, then you need to apply for an EIN number for the estate.
The decedent and their estate are separate taxable entities. Before filing Form 1041, you will need to obtain a tax ID number for the estate. An estate’s tax ID number is called an “employer identification number,” or EIN, and comes in the format 12-345678X. You can apply online for this number. You can also apply by FAX or mail; see How to Apply for an EIN.
https://www.irs.gov/businesses/small-businesses-self-employed/deceased-taxpayers-filing-the-estate-i...
Generally, the year a person passes away, you do a final personal tax return (1040, 1040A or 1040EZ) using the social security number. This return would include any personal income such as social security, pension income, 1099s or W-2 income before the date of death. It should include items which would have been reported by her personally and not estate items. You can pay the taxes from her estate account.
In addition to the final personal return, you may have to file estate return if there is an estate or a probate was opened. Items included on that return would be things that are the responsibility of the estate or the beneficiary.
