functions.php

add_action( 'manage_wc_booking_posts_custom_column', 'wc_custom_bookings_columns', 3 );
function wc_custom_bookings_columns( $column ) {
    global $post, $booking;

    if ( empty( $booking ) || $booking->id != $post->ID ) {
        $booking = get_wc_booking( $post->ID );
    }

    switch ( $column ) {
        case 'customer':
            $customer = $booking->get_customer();
            $user     = get_user_by( 'id', $customer->user_id );

            if ( $user ) {
                echo ' (' . $user->data->user_login . ')';
            }

            break;
    }
}