As an L-2 spouse she is employment-authorized incident to status, so immigration does not bar her working. Nursing in the United States additionally requires state licensure, which is a board process separate from her immigration status.
Immigration permission and professional licence are two gates
Hypothetical example: the wife of a Rothesay validation specialist is an experienced registered nurse and assumes that once she can legally work she can take a hospital position. Her L-2 status will authorize employment incident to status, which removes the immigration obstacle. It does not remove the licensing one: the state's nursing board will require an application, credential verification and possibly an examination before she can practise, and hospitals generally will not hire without the state licence in hand.
She should start the board process well before the move, because verification can take months. If she instead intended to seek TN in her own right, the Registered Nurse listing accepts a state or provincial licence or a Licenciatura Degree, and the same board process could still arise for lawful practice. Their children under 21 may accompany and study but may not work.
Planning her licensing timeline alongside his petition avoids a long period of unintended unemployment.