You have two options for entering an order for your customer:
1) Turn on user impersonation for your account.
a. Go to Preferences > General Settings > General tab.
b. Click the box marked "Allow User Impersonation" and set a site master password. Save.
c. Go to Sites > [site name] > Site users
d. You will see a green arrow next to each user's name. Click this arrow to log into the site as the user.
e. Place the order.
(Please note: You cannot edit/update a site users profile information by logging into their account via Impersonation).
2) Reset the user's password and login as the user.
a. Change the user's password in Sites > [site name] > Site Users > [site user] > General Information tab, Save.
b. Login as them and place the order.
c. Tell the user their new password (they can change it).