If an US greencard holder remains outside of the United States continuously for more than one year, the Department of Homeland Security (DHS) takes the position that residency has been abandoned. In addition absences from the United States for a period greater than 6 months but less than one year breaks the continuous residence requirement for purposes of naturalization. I would encourage you to gather all documentation reflecting the need for your mother to be outside the United States for more than 6 months such as (death or ilness in the family, emergency, etc.). There are several factors that must be considered before you mother is deemed to have abandoned her residency but I would caution your mother regarding her travels. In addition, if she anticipates to travel extensively, she should consider applying for naturalization.