What language is your web application written in? Will your client be hosting it on their server or are you hosting it for them?
In either case, you need to install the Crystal XI runtime on the webserver where the application is running. If your application is in ASP.NET, you should have (or be able to download) the Crystal XI Merge Modules to install. You may have to enter your Crystal Developer license key as part of the installation, but I'm pretty sure that's part of the "redistribute" license for applications that use Crystal so it should not be a problem that your client doesn't have an XI license.
-Dell