When you create a new user, that user gets their own home directory, exactly like the home directory you see when you log-in with your main user. Any domains or sub-domains you create under the control of that user will be placed in that users home directory, not your main users home directory.
When your friend logs-in using FTP and the new user credentials, they will only see the directories related to the domain(s) they control, they will not see any of your domain directories.
You can log-in using FTP to your friends home directory, but you will need to use the log-in details for your friends user.
I hope the above makes sense to you.
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