Hi,
How is she completing the order on behalf of the customer ?
If it's from the backend, after clicking on the "new" button on the orders listing, she can select the user to attach to the order and there is a dropdown there to automatically add the address of the customer for the billing and / or shipping address of the order.
If it's from the frontend, then she would need to use an extension like this one (
extensions.joomla.org/extension/shack-user-switching/
) in order to login as the other person and do the checkout normally with the user account of that other person.