Make sure the co-author has a FormsCentral account (co-author can have a free account). Once the account is created then the co-author should see the form in their FormsCentral dashboard. You can then use the co-author email address for the email receipt. When you add someone as a co-author they should receive an email about it. You can ask you co-author to look in their spam folder if they have not receive anything.
Can you please send me (firstname.lastname@example.org) the email address of your co-author and approximately when you added them as co-author. I'll verify on our end if the email was ever sent.