Can you embed one SharePoint web part into another? For example, a client wants a document library but also an ability to write an executive summary about that document using the rich text editor tool. Can this be done?
I'm not 100% sure I'm answering your question, but I did this by building my base site in a wiki library, which gave me the rich text stuff. Then I embedded other elements into the wiki page as I needed. So, in my case each page is dedicated to a network site. The main text is about that site, and then I embedded a site photo library, a doc library dedicated to that site, a task library dedicated to that site, etc...
You can create a new Site Content Type by going to Site Settings > Site Content Type Gallery > New Site Content Type. Create new type that has a parent content type of Document. Then use the Add From New Site Column button to define the columns for the summary. Finally create a new List on the site to display your new type and stick the list in a web part.
You cannot place one Web Part into another.
The solution for your client might be a custom content type as Jeremy said. The you can add custom fields to your content type like summary as a multiline text column with rich text support.
In your document library you have to enable content types and attach your custom content type to the list and set the content type as default content type. If your client needs lots of lists using the custom content type you might save your list as a list template to make it reusable.
If your save a document to the list using Word the user will get an option to fill out the content type fields.
A simpler approach would be just to create a new column in your document library, with the type "Multiple lines of text" with the formatting of Rich Text.