Sync option is missing for the shared folder B2B

When I share a folder with my partner (another M365 tenant) the person I share the folder with can open it. Before he is asked for a code which is sent to him by mail.

But when the folder is open in the browser, there is no Sync option to sync the folder using the OneDrive client. 

When we looked at my partner's OneDrive website, the sync button was also hidden under the Settings (gear icon) on the page. 

Any tips on how to make it work and enable my partner to sync the shared folder to his local computer? 

Thanks in advance 

Michal