According to the NY page on Exemptions for Persons with Disabilities, she must either qualify for SSDI or SSI, or one of a few other programs. She must be able to submit one of these forms:
- Award letter from the Social Security Administration certifying the applicant's eligibility to receive Social Security Disability Insurance (SSDI) or Supplemental Security Income (SSI)
- Award letter from the Railroad Retirement Board certifying the applicant's eligibility to receive railroad retirement disability benefits
- Certificate from the State Commission for the Blind and Visually Handicapped stating that the applicant is legally blind
- Award letter from the United States Postal Service stating that the applicant is certified to receive a United States Postal Service disability pension
- Award letter from the United States Department of Veterans Affairs stating that the applicant is entitled to a veterans disability pension
She must also have an income less than $29,000 per year, including her social security income. Different municipalities may have other restrictions (including a lower income limit), or may expand a smaller break to people above the limit, but you'd have to check with your local revenue authority for their specific rules.