Custom function for specific user role

The core has functions like current_user_can to check for an capability of a role or check the role directly. The follow example extend your source for this check so that the function not (!) run, if the current user haven’t the rights, the capability of his role.

add_filter('request', function( $vars ) {

// Check the capability of the current user.
if ( ! current_user_can( 'manage_options' ) )
    return;

global $typenow, $wpdb;

// Add logic to determine if the orders list should be filtered, and which
// currency
$currency = 'GBP';

if($typenow == 'shop_order') {
$vars['meta_query']['relation'] = 'AND';
// Only show orders in the specific currency
$vars['meta_query'] = array(
    array(
        'key'     => '_order_currency',
        'value'   => $currency,
        'compare' => '=',
        ),
    );
}
return $vars;
}, 15 );