No. TD status permits no employment, and running a business, even remotely for Canadian clients, is work. He would need his own status; Accountant is a listed TN profession if he holds the required degree, but self-employment is excluded there too.
Remote work is still work in TD status
TD dependants may live in the United States and study, but they may not engage in employment, and the prohibition is not limited to U.S. employers or U.S. clients.
Continuing to invoice Saskatchewan clients from a home office south of the border is employment in the United States for immigration purposes and puts his status, and potentially hers, at risk. His options are his own. If he holds a bachelor's degree in accounting or a professional designation, Accountant is a listed profession, but TN would require a U.S.
employer; he could not enter TN to run his own practice. He could wind down the practice and later seek employment, or the family could consider whether a different principal category with a working spouse fits better. Children in TD may attend school and cannot work, and they lose the status at 21.
Decide his plan before the move, because an unplanned home office is the commonest way a TD spouse falls out of status.