yes, add another text box to the form, place your cursor where the text box should go, the Go to insert -> Forms -> Text Field.
name it something like "freindsEmail"
then edit this code:
$RecipientEmail = "".($WAGLOBAL_Contact_Email_To) .", ".(isset($_POST['UserEmail'])?$_POST['UserEmail']:"") ."";include("WA_Universal_Email/WAUE_contact_1.php");
to use the new texxt box as well:
$RecipientEmail = "".($WAGLOBAL_Contact_Email_To) .", ".(isset($_POST['UserEmail'])?$_POST['UserEmail']:"".", "..(isset($_POST['freindsEmail'])?$_POST['freindsEmail']:"") ."";include("WA_Universal_Email/WAUE_contact_1.php");