This is going to take some involvement/help from support to deal with, as issuing "new" credentials for a domain/account is a very sensitive issue. The account belongs to another, and they should not be granting access to that account to a third party without the participation of the account holder. This is a "Good Thing"(tm) for lots of reasons.
The domain registration information determines who has a right to use the domain; but this is not related to the management of the hosting account at all.
First of all, if the domain was hosted on DreamHost under another's account, it would not be appropriate for DH to allow "Control Panel" access to that account for a different person. This complicates matters further if the domain's ftp user is the missing webmaster's, as file/directory permission may be structured in such a way that to allow another to access those directories via ftp/sftp could expose the original account holder's files.
In this situation, what really needs to be done is for your friend to obtain her own account, using her own credentials, and to lobby for DreamHost to "move" the domain in question to her account. Generally, due to potential promo code fraud, they will not do this, though it has been reported that exceptions have been made in special circumstances. I know you said she wanted to avoid having to have her own hosting account, but if she wants control of her site, that cannot be avoided - as long as someone else control access to the site's file, she will not have control (as you have just observed)
For there to be any chance of this happening, your friend will have to be the registrant or administrative contact of the domain in question (otherwise, such a process would be tantamount to a domain hijacking!). If she is not the registrant of record (see the whois information fro the domain), that needs to be taken care of first, so that your friend has "standing" to initiate discussion with DH on this issue.