SSI vs SSDI
SSI in a low income eligibility program as PP said, if you do not meet the quarters. If they retired on their quarters they were on SS benefits, if she retired disability than it was Soc. Sec. Dis Benefits., if she had her quarters.
You can receive both SSI IF you become disabled and your quarters are lower than that of the SSI low income levels. For example, you are eligible for soc sec disability, but you had a low paying job and you have no other income source, it may be supplemented with the SSI portion.
The office can explain it better, but if she is qualified as SSI, I don't think the filing or disability would increase her benefits as the SSI should be he secondary and most she would receive.
What about her age? Can she file as a widow or off spouse or her ex's benefits if he reached 65/66 years of age? I think this maybe the only way she may have an increase, but really there are so many variables as ag you need to have her call with a list of questions. Also, disability benefits are really difficult to receive, I would be confused to why she would try for disability if already receiving the most she could? Baring the collecting off spouses benefits.
Wishing luck to her,