This is a simple way to change an addon domain user password in cpanel. 1) Login into cpanel 2) Click FTP Accounts or Find FTP accounts or FTP Manager 3) You will see a page with a list of your addon domains with that particular account. 4) You can change your user password from change password option there. 5) Make a new site with FTP 6) The website will be whatever.com 7) The username will be email@example.com/ca/org/etc
For example, a website called mysite.com hosted in the master account mymasteraccount.com would have a username firstname.lastname@example.org 8)Now, the client could login from any FTP software with those authoritative credentials.