Your mother can go to the local CBP Deffered Inspection sites to see whether the CBP officer can fix the I-94 for her. Here is the website to find a CBP Deffered Inspection Office near your area:
She also can go any port of entry to fix the I-94. Find a port of entry at your area at this website:
http://www.cbp.gov/xp/cgov/toolbox/contacts/ports/
If she can not fix the I-94 at the CBP office, she may file I-539 to extend the B2 status before the I-94 expires though the extension may be difficult to get approved.