At one level, she should tell her mother to never use her SSN again, and ask her mother to cover her losses -- but it doesn't sound like mom is very responsible, or likely to have the money.

At another level, the woman herself may be under investigation for fraud, if her earnings would have disqualified her from the public assistance her mother received in her name. Also, even if not, it may be that she continues to have tax refunds taken until the amount of assistance received by her mother is repaid. It may not be possible to extricate herself from this legal mess without turning her mother in, or cooperating with the prosecution of her mother.

It's an unfortunate situation and one from which she may not be able to protect her mother; her mother may have to face the consequences of having chosen to commit a crime while using her daughter's identity.