Move the message inside your IF statement so it's only executed IF add was passed.
if( ! empty($_GET['add']) && 0 !== (int)$_GET['add'])
$result = mysql_query(sprintf(
"INSERT INTO dbFriends (my_id, friend_id, status) VALUES ('%s', %d, 0);",
echo $result ? 'Success' : 'Failed' ;
*empty probably considers 0 as empty, but you'll need to check; hence the additional conditional.