In messaging_simple module page the user name is displayed, but not using the theme('username') function. This breaks Realname re-writing of user username.. the edit is simple and done in messaging_simple_user_page function replacing the function first lines with this one:

function messaging_simple_user_page($account, $op = NULL, $id = NULL) {
  drupal_set_title(t('Messages for %name', array('%name' => theme('username', $account->name, array('plain' => TRUE)))));