My mother has a visa to visit the US from overseas. The visa is valid for 5 yrs, multiple entries and for stays of up to 6 months at a time. Does driving to a border town just inside Mexico (say Tijuana) and then returning to the US reset the length of her current stay? In other words, if she were to be here 4 months and then take a day trip to Tijuana, would she be able to stay for 6 more months starting when she returned from Mexico at the border crossing?