WooCommerce has a very broad documentation, one topic in it is about Customizing checkout fields using actions and filters, you can easily set billing_email
to not be required. (code untested)
add_filter('woocommerce_checkout_fields', 'WPSE0309_woocommerce_checkout_fields');
function WPSE0309_woocommerce_checkout_fields( $fields ) {
$fields['billing']['billing_email']['required'] = false;
return $fields;
}