Unfortunately, that is an "all too common" circumstance in the web development business.
First, try the ftp user/pass you were given on the panel. It is possible that they are also a "web id" and will work.
If they don't, assume the "worst" and immediately log in via ftp and download everything you can find, in every directory, to your own computer. If there is the slightest possiblility that there may be "issues" between the previous developer and the owner, you want to make sure you preserve the files that are there, in the event they should suddenly "disappear". In fact, you should do it anyway, even if you don't feel this is likely to be a problem (far better to be safe than sorry!).
Having done that, check with the owner to see if they have a web-id for the Dreamhost account management Panel. You didn't say whether the developer, or the owner, controls the DH account. If the owner controls the account, he should take the steps I described in my previous post, or give you sufficient credentials so that you can do it (regarding password controls, etc.).
In the worst case, if you are able to grab the files, you should be ok. It may be that the previous developer is hosting the owner's site under his (the developer's) account on DH, and the owner only has an ftp account. This is not that unusal, and is not necessarily a problem, as long as you obtain all the files, and after you have done this, the owner prevails upon the previous developer to "drop" the domain from his DH account so that you, or the owner, can host it under a second account.
Hopefully, they can work this out themselves, but if not, the owner will likely need to initiate some discussion with DH support to get it sorted. As long as the domain is "already" hosted on Dreamhost, and the DNS is pointed to Dreamhost DNS servers, you will not be able to set that domain up in your "own", or the owner's, account without intervention from DH.
Does that make any sense?