Your mother should talk to a consumer lawyer as soon as possible. Based on your response, it appears your mother probably has a judgment against her. It is possible she was never properly served with the lawsuit and the judgment can be overturned. If she just ignored the lawsuit, it is unlikely anything can be done about the wage garnishment.
There also seems to be a question about whether payments are being properly applied. I would call a consumer lawyer in your area. You can find consumer lawyers by going to http://www.naca.net/
It sounds to me that she needs to contact a consumer attorney. There are several excellent attorneys in this area who do this type of work.
There are a couple of issues at play here; 1) the possible conduct of the debt collector (Fair Debt Collection Practices Act places strict rules on debt collectors) and 2) The judgment for garnishment.
She should probably request that the debt collector send her exactly what is owed in writing. While she is negotiating with the debt collector it would be unlikely that he will continue with the garnishment (Don't get me wrong--he might...but I can tell you from experience, it's a real pain). Also, keep this in mind. Some debts are purchased b y the debt collectors. That means that sometimes they MAY be willing to accept less.
My advise is this: Go talk to an attorney!! And, do so quickly. They can get more information from you and give you right advice. Also, just having that attorney call for you changes the entire tone of the conversation with the debt collector.
PLEASE NOTE: This "Answer" does not create an attorney-client relationship. I am NOT your attorney. Also, my law firm has NOT been engaged to represent you in any capacity.
The response is in the form of legal education and is ONLY intended to provide general information about the matter within the question. Oftentimes the question does not include significant and important facts and timelines that, if known, could significantly change the reply and make it unsuitable.
If you are unclear about this or any of the content in this "Answer", I would encourage you to seek the advice of legal counsel.