The easiest thing to do in that situation is to change her nameservers over to ns1.dreamhost.com, ns2.dreamhost.com and ns3.dreamhost.com. This enables you to use the DreamHost control panel to manage the DNS entries for the domain.
Another possible choice in the “easy” category is to change the actual domain registration over to DreamHost. DreamHost makes you renew the domain for a year when you do this, so you’ll have to pony up the $9.95 for that if you choose to do it that way.
Finally, the straightforward-but-not-as-easy-as-the-easy-way way is to add the domain to the DreamHost control panel as a managed domain and then to copy over the IP address from the DreamHost DNS page for that domain to the flowersbydianemiami.com and ftp entries. As you mention, you should also copy over the mail server entry from the DreamHost panel to the Earthlink panel. Note that you’ll have to update the IP address manually should it ever change at DreamHost.
The WWW entry can be left as-is (though I personally think it is potentially as smidgen slower in theory though not in practice in your case for reasons I won’t bore you with).
Not sure what the localhost entry is for (testing, maybe?), but I’d guess it’s safe to leave it as-is.
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