She could have filed as Married Filing Jointly in 2017, the year of her spouse's passing. To have Qualifying Widow filing status she must have a qualifying child or step-child as a dependent.
IRS Publication 501 Dependents, Standard Deduction, and Filing Information page 9 - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=9
For tax year 2018, if she does not have a qualifying child or step-child as a dependent, then she can only file as Single and not Qualifying Widow.
If she is under the age of 65 then her standard deduction is $12,000. If age 65 or older the Single standard deduction is $13,600 (2018).
If Social Security is the only income she receives, she does not have to file a tax return. Unless there are federal income taxes being withheld then she would file for a tax refund. Of if she has health care insurnace provided by one of the state marketplace exchanges under the Affordable Care Act (ACA) and received a Form 1095-A then she would need to file a tax return.
Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:
- Married Filing Jointly - $32,000
- Single or Head of Household - $25,000
- Married Filing Separately - 0
