If they are legally married, the spouse can be admitted in L-2 status and is authorized to work incident to that status. A common-law partner is not a spouse for U.S. immigration purposes and would need a category of their own.
Marriage, not cohabitation, decides the dependant question
Hypothetical example: the developer has lived with her partner in South Frontenac for nine years without marrying. Ontario treats them as common-law spouses for many purposes, but U.S. immigration law recognizes only a legal marriage for L-2 dependant status.
If they marry before the move, the partner can be admitted as an L-2 spouse, may work incident to that status once admitted, and may study. If they do not, the partner has no dependant route and would need to qualify independently, for instance through his own employment in a listed TN profession if he is a Canadian citizen with the required credential, or through a visitor admission that permits no work. This is a decision the couple should make on its own merits, but it should be made knowingly and early, since a marriage after the developer has already been admitted can still support a later L-2 admission for the spouse.
Any children of the developer may accompany her in L-2 status and attend school but cannot work.