Cannot access WordPress control panel, Ex-partner has all info

An ex business partner of mine built a website for one of our clients, but has since disappeared and I am unable to contact him. My client is now unable to access the WordPress dashboard of the site because they have forgotten / misplaced the password, and they cannot reset the password because we don’t know what email address my ex business partner used to set up the site. I don’t know what hosting provider he used, or what domain registrar he used, or anything. How can I get access to the username and password of the site now? This is really urgent, as my client is wanting to sue me for damages since they can’t update the website.