Thanks for looking into this, it confirms my diagnosis that the site is not configured for .NET 4. Changing it to .NET 4 in the control panel should fix the problem and there is no need to edit Web.config.
The issue is that in .NET 4 some things that were previously needed to be declared in Web.config are now already declared in the .NET 4 version of machine.config and therefore cannot be declared in Web.config. So the reason for the error is that his site is using .NET 3.5 which does expect this section to be declared in Web.config, but the package he installed is for .NET 4 so it correctly does not have the system.web.extensions section declared in Web.config.
So, the correct solution is to change it to .NET 4 in the control panel.