How to resend username and password to user in WordPress using retrieve_password function

Well, there are no such method where you can resend your existing user a reset password from your side. they must actually click on the "lost password" link and fill up the form in order to retrieve the activation email to reset their password. The method was retrieve_password() which is located in wp-login.php. Hence, you won't be able to use this method and even if you can, you will need to post it over from a form in order to utilized this method. The most easiest way to reuse this method is to recreate it by removing certain condition so that it makes life easier for you.