Sometimes a customer will have trouble locating their order confirmation in their inbox. If so, you can easily resend them their email confirmation.
Start by locating the customer and the order number they want the confirmation for. Click on that order and then over on the right underneath Order Actions click the Resend email confirmation link.
Tip: It is possible that these emails end up in the customer's spam or hung up with their email server. If they don't receive it right away have them check their spam or wait a few minutes for their email serves to catch up.