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I'm not sure what the mechanism is, but the nurse is only partially right. An increase in pCO2 causes vasoconstriction in the pulmonary vasculature and vasodilation EVERYWHERE else. CO2 causes constriction in the pulmonary vasculature because an increase in pC02 sends the message there is damage/shunting here, sending blood here will not effectively oxygenate it so blood is preferentially sent to a different part of the lung where it will receive better oxygenation.
This results in a decrease in tidal volume causing alveolar hypoventilation and hypercapnia. Hypoxia and hypercapnia cause pulmonary vasoconstriction, which increases intrapulmonary resistance and shunting (Figure 34-11). This results in hypoperfusion of the lung and a decrease in effective pulmonary blood flow.