The nurse's spouse and unmarried children under twenty-one may seek TD status. TD family members may study but cannot work, and a child's derivative eligibility ends at twenty-one.
One income, for the time being
Hypothetical example: the nurse's husband drives a milk truck and their daughter is twenty. Neither can work in the United States as a dependant of a TN holder, however ordinary the work or however short the shift. The husband's options are a status of his own — a listed profession he qualifies for, an employer petition, or a student route — assessed on his own credentials rather than on his wife's offer.
The daughter may study, and probably should, because her dependent status ends on her twenty-first birthday and a course of study can sometimes be continued under a student status afterwards. Where a household depends on two incomes, that arithmetic belongs in the decision before anybody resigns anything. Keep one page listing each person's date of birth and intended activity, and revisit it before every extension.