Under the DreamHost system, disk space is allocated to users, not domains. The space allocated to the domain will be whatever is allocated to the user that controls that domain.
If you wish to restrict the space that a domain can use, I would suggest creating a new user, setting that users disk quota to whatever limit you require then when you add the domain be sure to specify the new user as the "FTP user / CGI-runs-as user:".
This will also mean that the domains web accessable directory will be placed in this new users home directory and when your friend logs in via FTP, using the new users credentials, he/she will only see the files related to this domain, not your other domains.
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