Basically you have the option between using IIS as the web server for QlikView or using the built in web server, called QlikView Web Server. The QlikView Web Server is purpose built for QlikView and I'm not sure if it's completely developed by Qlik from the ground up or using a lightweight distribution of IIS as a baseline web server that's configured for Qlik use.
If the client is wary about running a web server that they are not familiar with, why not just use IIS as the web server? They should be familiar with the IIS configuration and QlikView is very straight forward to set up with IIS when installing.
(The QlikView Server reference manual has a installation guide).
