Confirm there are not already settings or features that do what the shop is asking for:

Confirmed. There is currently no report or export option that ties an individual customer's name and order details directly to the item quantity ordered. While the updated "Order Fulfillment Detail — CSV 11" contains order and customer details, the quantity column was removed.

What does the shop want the product to do:

The shop wants the "Quantity" column to the CSV 11 ("Order Fulfillment Detail") report so they can see the exact quantity of each product ordered by a customer on a per-order basis across a specified date range.

Why/how would it improve sales or efficiency:

Operational Efficiency: Eliminates the need to manually cross-reference customer orders with separate inventory or fulfillment views to verify how many units an individual purchased.