I do not have the exact reference to the rules or FAQs where this answer comes from originally, but it can be found on the very reliable (and source providing) website Descent in the Dark, here:
The overlord controls the hero for that attack, including the hero's use of surges and power dice. The overlord may also play cards such as "Aim" with the attack. However, the overlord player cannot move the character, or force the hero to spend fatigue to add to the attack. The overlord may not force the character to use any orders. The hero retains control of any of her defensive options such as shields or Ghost Armor.
So, yes, the hero may still use his/her shield in the situation you describe.
Have fun! :-)