I believe what you want is basically a completely separate site, but the NAME is a sub site of the main site, like “friend.mysite.com” so he can treat it as if it was a new site, with it’s own public_html, etc.
That is easy. Just create it as a new site, but use "friend.mysite.com" as the name.
It will use one license count from your license, if you are using the pro version, because it is actually an independent site within Virtualmin.
It has everything, new password, new user (if you set a new user), email, etc.