$customers = $this->customerRepositoryInterface->getById(2);
       print_r($customers->getCustomAttributes('enable_alerts'));

Array (
    [enable_alerts] => MagentoFrameworkApiAttributeValue Object
        (
            [_data:protected] => Array
                (
                    [attribute_code] => enable_alerts
                    [value] => 1
                )

        )

)