Yeah, with domains registered elsewhere you simply need to configure them to use the DreamHost name servers, add the domain to the DreamHost hosting system and the jobs done.
However, if the domain is registered under a different account at DreamHost, it will already be configured to use the DreamHost name servers and I suspect that simply adding the domain to the hosting system under the other DreamHost account may not be possible, or if it is possible to add the domain, it may still not work, due to the accounts being on different physical servers.
Having said this, I am not absolutely sure it will not work, as I have never tried it. Perhaps the original poster can simply add his friend's domain to the hosting system via the panel at Domains -> Manage Domains -> Add New Domain/Sub-Domain and it will work as intended, it is definitely worth a try.
If it does work, I apologize to the original poster for misleading them, but it can't be that simple, can it?
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