An L-2 spouse is employment-authorized incident to status, so working for a Canadian employer while physically in the United States is generally within what her status permits. Tax and employer-policy questions are separate.
Status permits it; the employer may not
Hypothetical example: the spouse of a Quispamsis L-1B analyst works as a payroll administrator for a New Brunswick company that has agreed in principle to let her work remotely. Once admitted in L-2 status she is employment-authorized incident to that status, which covers work for a foreign employer performed in the United States. The immigration side is therefore simpler than many families expect.
What remains are non-immigration questions: whether the Canadian employer is willing to have an employee resident in another country, how payroll withholding and tax residency will be handled and whether her professional designation has any location condition. She should obtain the employer's agreement in writing and take tax advice before relying on the income. The reviewer should be told that she intends to work so that her L-2 documentation is prepared with that in mind.