You co-worker lives in WY? Not having children in WY has a much bigger impact on reducing carbon due to their high per-capita carbon output - having no children instead of the two children in your SF example will save 200 tons of carbon per year.
She and her husband would have to reduce their carbon to 0 in order to achieve the same 200 tons per year savings.
You are absolutely right - reducing population where there is high carbon-per-capita rates has bigger impact on carbon than in low carbon-per-capita rate areas.