Laravel Forge + DigitalOcean - Adding a SFTP user with restricted access only to a directory

I am using Laravel Forge in a DigitalOcean droplet. I need to grant access to a specific directory on the server to one of my clients. He needs read/write access to only that directory, and I am having problems with the setup.

I have followed the steps in http://ift.tt/2fCnVkM, but I am stuck because Forge does not permit password authentication, only through public/private SSH keys...

Has someone done something similar? What would be the best approach for getting it done?

Many thanks in advance.

via Chebli Mohamed