Tools like Fantastico and Installatron help the end users to easily install a good number of popular and free applications, including forums. I guess this is what that customer was looking for.
These applications, as I said, are free, and anyone could go to say phpbb's site, download the package and then install it on his hosting account.
Few hosting companies truly support these applications though, inthe sense of guiding the customer in how to install them, troubleshoot them etc. It can easily take a newbie customer hours to understand the application, how it works etc. Hosting is simply too cheap to suport the level of hand holding that such a customer would require.