She would probably need to file based on her husband's pension checks.
If Social Security was the only income she received, then your gross income equals zero, and you don't have to file a federal income tax return.
The minimum income amount depends on your filing status and age. In 2020, for example, the minimum for single filing status if under age 65 is $12,400. If your income is below that threshold, you generally do not need to file a federal tax return.
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When Does a Senior Citizen on Social Security Stop Filing ...
Publication 554 (2020), Tax Guide for Seniors | Internal ...
