Wow! This is so sad. Your mother's social security benefits are not subject to garnishment (unless this was for a student loan, possibly). She needs a consult with a local consumer protection attorney who likes tangling with debt collectors, because this debt collector has overstepped his bounds by upsetting you to the point where you ask how to help her.
If he was collecting from income that was not Social Security, that would be different, but if she did not realize that her SS cannot be garnisheed he should have stopped if she told him that was her only income. A debt collector cannot rely on a defense that a defendant "allowed" the collection, that is not reasonable.
Google NACA debt defense for Spokane (NACA is National Association of Consumer Attorneys); there was an article in the paper within the last two months about a lawyer who deals very effectively with debt collectors. I hope you find him and that you can get your mother in for a consult.
No promises here, but the kind of lawyer who takes on cases like this understands that people who are living on social security less a garnishment are not exactly wealthy and such a lawyer may well be able to come up with a different arrangement to get paid. Please don't be afraid to ask.
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If your mother’s only source of income is SSDI then they cannot garnish her benefits.
DISCLAIMER: David J. McCormick is licensed to practice law in the State of Wisconsin and this answer is being provided for informational purposes only because the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship.
David is right, the SSI is exempt from execution (collection on the judgement) - but she might want to look in to Chapter 7 bankruptcy if she wants to wipe out the judgment altogether.
Consult a local bankruptcy attorney.
Clark County, Nevada practitioner.