add_action( 'restrict_manage_posts', 'shop_order_user_role_filter' );
function shop_order_user_role_filter() {
	global $typenow, $wp_query;
	if ( in_array( $typenow, wc_get_order_types( 'order-meta-boxes' ) ) ) :
		$user_role	= '';
		// Get all user roles
		$user_roles = array();
		foreach ( get_editable_roles() as $key => $values ) :
			$user_roles[ $key ] = $values['name'];
		endforeach;
		// Set a selected user role
		if ( ! empty( $_GET['_user_role'] ) ) {
			$user_role	= sanitize_text_field( $_GET['_user_role'] );
		}
		// Display drop down
		?><select name='_user_role'>
			<option value=''><?php _e( 'Select a user role', 'woocommerce' ); ?></option><?php
			foreach ( $user_roles as $key => $value ) :
				?><option <?php selected( $user_role, $key ); ?> value='<?php echo $key; ?>'><?php echo $value; ?></option><?php
			endforeach;
		?></select><?php
	endif;
}
add_filter( 'pre_get_posts', 'shop_order_user_role_posts_where' );
function shop_order_user_role_posts_where( $query ) {
	if ( ! $query->is_main_query() || ! isset( $_GET['_user_role'] ) ) {
		return;
	}
	$ids    = get_users( array( 'role' => sanitize_text_field( $_GET['_user_role'] ), 'fields' => 'ID' ) );
	$ids    = array_map( 'absint', $ids );
	$query->set( 'meta_query', array(
		array(
			'key' => '_customer_user',
			'compare' => 'IN',
			'value' => $ids,
		)
	) );
	if ( empty( $ids ) ) {
		$query->set( 'posts_per_page', 0 );
	}
}