Dropbox access permission

I want to use Duplicati to backup two computer to my Dropbox, because I have plently of space on it.
One of the PC is owned by a friend of mine, if I authorize Duplicati on his computer to backup on my Dropbox does Duplicati will have access to all Dropbox account or only to his backup folder? Are other data on Dropbox account accessible from that computer?
I do not want to let him access my personal data even if his computer is going to be backed up on it with Duplicati.

Thanks for support!
Best Regards