We are maintaining SharePoint as a file server. My management asked me to add one of our external client to invite our SharePoint site and let him sync the file to his PC. I was able to successfully invite him to the site but he cannot able to sync the SharePoint site to his PC using OneDrive application. When he click sync button in SharePoint online nothing happens
Syncing with an external or guest account" is currently not supported by OneDrive. Please check this uservoice request Allow external OneDrive for Business users to sync folders.
So, there is nothing you can, if you decided to share with an external guest they have to manually upload and download the file through online.
Hope this helps
You are right. It is very disappointing to see as a limitation on a Microsoft Product. I don’t get the Microsoft intention on it. Anyhow I have voted the uservoice and subscribed the post we will how long will it take for Microsoft to remove the limits.