There is nothing to guarantee that; at DreamHost they have machines in clusters and so users are spread throughout a cluster.
Generally the user doesn't matter; think of it as a passcard, as long as it works you're ok, it doesn't matter if it says fluffybunny or ricosauve on it. So adding and deleting users is a bit hasty and probably pointless. The more important thing is whether or not that user gets access to the services.
So you got your own DreamHost ID now? Do you just get an ID or did you sign up for hosting services too as a new DreamHost customer? In otherwords, who is paying DreamHost - you or your friend? This makes a difference - customers can grant permission to others to access the web panel OR if someone signs up as a customer themselves Support has to get involved if the domain registration and/or hosting needs to be transfered to the new customer. As for actually user files and mail, you can ask support if they can transfer those for you but most likely you would have to transfer those yourself once the services are working.