Their site is probably not an ASP site since they are using Apache Tomcat (Apache Tomcat is the servlet container that is used in the official Reference Implementation for the Java Servlet and JavaServer Pages technologies. The Java Servlet and JavaServer Pages specifications are developed by Sun under the Java Community Process.)
So, if you are still interested in the work - there are many ways for a system to be developed that allows for management of the content - which is typically called a Content Management System (CMS). If the system is completely "hard-coded" at this time, and there is no foundation for configuration of the site's content - then everything will probably have to be re-written or changed a great deal.
One approach is to have a completely data driven site where all the configuration and content is stored in a database (or some other system such as a hierarchy of folders containing XML files) and the html for each page is generated dynamically for each request. In this scenario it is not uncommon to cache some of the data or generated html to improve response time. All content, links, nav-bars, form-handling logic, and so on are gathered and dealt with on a request-by-request basis - the application determines what is being requested and acts on it.
Another approach is to regenerate the entire site whenever content is pushed to production. In this scenario, everything could still be stored in a database or other repository, but rather than dynamically generating pages, all the html is generated (with all the proper links, navigation, etc) for the entire site at one time. The main advantage of this is the performance in requesting a page will typically be much better than a dynamically generated site, and will therefore scale better as well. You might call this a "one-click" or "pre-generated" site.
Sites like this forum are usually dynamically generated due to the nature of the content (that is, it is frequently changed in "real time". Of course, your client more likely has a brochure-ware site or an online store, both of which could be implemented with either a dynamic or "pre-generated" site.
This is a huge topic. The best way to do this for your clients needs could be one or the other, or a combination of both of the ways above - or some other way all together.
Often you will also use a combination of CSS and various customizable templates to allow your client to change (or re-skin) the site's look and feel.
There are many CMS systems commercially available. I have worked with several on various projects. They were much too "general" in nature and were therefore overkill for the projects I worked on. A few were also very complicated and poorly designed. It is likely there are better ones available now, but they can be very expensive (easily $50,000+ US) so you have to do a good job of investigation before making a decision.
Have fun!
Woody Z
http://www.learntoprogramnow.com