Does the previous host use cPanel? If so, our importer tool in the web panel can probably just grab everything for them and set it up here automatically: http://wiki.dreamhost.com/CPanel_Importer
If not, things will need to be brought over manually. Unfortunately there is not a real easy way to bring email accounts over, but she will indeed be able to still use any current addresses she deems important. Any actual emails/messages currently on the server that she'd like to keep should be backed up/downloaded locally via a mail client such as Thunderbird or Outlook. Once the domain is added to your account here (on the Manage Domains page in your panel) to be hosted, you can go to Mail > Manage Email and re-create the important addresses for the domain there. Alternatively, you can have our panel help set up Gmail for the domain, and the addresses can be set up there as well.
Once the addresses are set up and you have all the site content uploaded to our servers, you can go to where the domain name is registered, and point the nameservers to ours: ns1.dreamhost.com, ns2.dreamhost.com, ns3.dreamhost.com.
Also, she can transfer her domain registration to us if she'd like at any time; it will not affect her current registration other than by adding another year to the current expiration date!
Here's another article that might help: http://wiki.dreamhost.com/Transfer_your_hosting_to_us . And please feel free to contact our support DreamTeam if you have any questions or need help setting any of this up: http://dhurl.org/dhsupport
Hope this helps!