EDIT: ahora sí
Código PHP:
$result = mysql_query("
SELECT DISTINCT *
FROM `muro`
LEFT JOIN `relationship` ON (`muro`.`usuario` = `relationship`.`to`) AND (`relationship`.`from` = '".$_SESSION['k_username']."'
OR `muro`.`usuario` = '".$_SESSION['k_username']."')
ORDER BY `muro`.`id` DESC
", $link) or die(mysql_error());
Gracias !!