Rental and Social Security disability benefits

This depends on if she is on SSDI or SSI.

The Social Security Administration is responsible for two major programs that provide benefits based on disability: Social Security Disability Insurance (SSDI), which is based on prior work under Social Security, and Supplemental Security Income (SSI). Under SSI, payments are made on the basis of financial need.

Social Security Disability Insurance (SSDI) is financed with Social Security taxes paid by workers, employers, and self-employed persons.

Note that SSDI payments may be reduced by worker's compensations. See this page (which does not reference rental income) for details :
http://www.ssa.gov/pubs/10018.html

Supplemental Security Income (SSI) is a program financed through general revenues. SSI disability benefits are payable to folks with limited income and resources. SSI benefits will most likely be reduced due to income from other sources.