A position governed by a collective agreement has employment conditions as well as immigration conditions. Ask the employer about the actual hiring process, then verify the spouse’s own L status before accepting a start date. Neither the agreement nor the principal’s petition supplies that status.
Check the evidence used for the spouse’s proposed start
An eligible L spouse is employment authorized incident to valid status; the appropriate spouse-specific I-94 is commonly relevant evidence. Keep that evidence separate from seniority, licensing or other conditions of the offered position. The employment arrangement cannot confer permission on someone without the required status.
An unmarried child younger than 21 may qualify for L dependent admission, but that child has no corresponding spouse employment benefit. Two permissions have to exist at the same time, and neither creates the other. The immigration side asks whether the spouse holds valid L dependent status and can evidence employment authorisation incident to that status, commonly through the appropriate spouse-specific I-94.
The employment side asks whether the posting may lawfully be offered to that person under the agreement governing it, which may reserve the vacancy for internal candidates or impose a licensing or seniority condition entirely unrelated to immigration. Hypothetical example: a spouse is offered a covered laboratory position, the employer confirms the internal posting rules were satisfied, and a start date is set before the dependent I-94 is checked. Confirm both in writing before a resignation letter is sent anywhere, and keep the two confirmations in separate files, because they will be read by different people for different reasons.