How can my mom become a legal resident after overstaying her work visa?

If you are anywhere close to the age of 21, your mother can benefit once you turn that age and apply for her permanent residence. She would then be eligible to adjust status to permanent residence without leaving the country as she entered the U. S. on a legal visa. Another option is to wait to see what happens with comprehensive immigration reform which is presently heading into the House of Representatives for an anticipated showdown with the Republican majority. The bill which just passed the Senate, S. 744, would allow most undocumented immigrants like your mother the ability to work and travel under registered provisional immigrant status and eventually a green card in 10 years. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.  

She could apply for a provisional waiver of the overstay bar, but that would require proving extreme hardship to you if she were deported.