The site is one I made a few years ago using Joomla and Easyblog. The owner now wants it redone as a WordPress site and the new developer is having trouble importing the 500 odd posts into the new WP site.
Ah, if that is the case, you can try to request them to pass in the "limit" in the query string when rendering the RSS feed (They should simulate how the pagination works when clicking on the pages)