An L-2 spouse is authorised to work incident to status, so she may work for a US employer or for herself in the United States. Continuing to serve Canadian clients from US soil is work performed in the United States and is covered by that authorisation, though tax and professional-registration questions arise separately.
L-2 work rights are broad; the practical questions are elsewhere
Since USCIS recognised L-2 spouses as employment-authorised incident to status, the spouse does not need a separate employment authorisation document to work, although many obtain one for employer convenience. Hypothetical example: the manager's wife runs a small bookkeeping practice for Lake Country growers and wants to keep it while living near the Washington plant; nothing in her L-2 status prevents that, whether she serves Canadian or US clients. The issues that remain are not immigration issues: whether her professional body allows practice from abroad, how her income is taxed in each country, and whether she wants to form a US entity.
The children, by contrast, may study but not work, and any child who turns 21 during the assignment loses L-2 status on that birthday and needs an independent basis to stay. The spouse's rights therefore should not be confused with the children's, which are narrower and time-limited.