If they only get SS benefits it is not taxable and they don't have to file a return. If they have other income they might have to file a return and then the SS might be taxable.
Up to 85% of Social Security 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
