You will need to use her taxpayer identification number on her final return. Be sure to complete the section under personal information that says you are preparing the return for someone who passed in 2023.
When you are preparing the tax return for the person who has passed, you will only file an individual tax return up until their date of death. Any income earned after the date of death would be reported on a trust return. Please see this link for more details.
I'm very sorry for your loss.