@David pointer to Anne-Marie’s article is a good read – read it to the end, there is good stuff there.
@collywolly suggestion of A Designer’s Guide to Adobe InDesign and XML is also good, though the focus is on getting XML into InDesign rather than out of it. It is the only indepth resource I have found dealing with InDesign and XML.
In order to find the right process it helps to ask for or look at the specifics of their website. If they have a fairly developed Web site contained in a Content Management System, like Drupal or WordPress, you may have to break up the content you are adding into bits and won't need to export complete HTML pages or on the other hand, you might need to add class styles to the HTML tags you create, in which case XML transformed by XSLT may be the way to go.
Taking time to think it through will save hours later. If they have a sympathetic web developer who is not threatened by your possibly taking work off their roster, making friends and getting their help is also a very good thing . . . There is more than one way to skin a cat, and on the web it could be a cougar, a chipmunk or a great dane, etc.