An event attendee may ask you to resend their confirmation email. You can send it through the event dashboard, or they can look it up themselves on the event homepage. 

To resend it through the dashboard, go to Dashboard>>Reports>>Ticket Purchases and search for their transaction. When you find it, select Manage. Right below the purchase number, there is a link that says "Resend Confirmation Email". Click on that to resend the confirmation email.