the center php page uses a loop to go through each form field that is posted and write its value, the label uses the form elements name, you edit the code that writes the form label to change "Contact Me group First Name" to be just "First Name"
The code in the center.php page to write the label will look like:
<th style="font-size: 10px;font-family: Arial, Helvetica, sans-serif;width: 134px; text-align: right; padding: 3px 10px 3px 3px; color: #FE6C00; font-weight: bold;"><?php echo(str_replace("_"," ",$pkey)); ?>:</th>
you can change it to:
<th style="font-size: 10px;font-family: Arial, Helvetica, sans-serif;width: 134px; text-align: right; padding: 3px 10px 3px 3px; color: #FE6C00; font-weight: bold;"><?php echo(str_replace("_"," ",str_replace("Contact_Me_group_", "", $pkey))); ?>:</th>
This will strip the "Contact_Me_group_" string from the form elements name leaving only "First name"