Domains are assigned to a specific user account and cannot be accessed over FTP by any other users. Your only options in this case would be to provide this person with your FTP login, or re-assign that domain to a brand new FTP user. The latter option would mean that the domain is only accessible by that newly assigned user, though. But, that would be the best way to protect the data for the other domains assigned to the original FTP user.
-DreamHost Technical Support Supervisor