It probably uses a web extension that the iPad does not support, like Flash, Java, or Silverlight. I can't tell without trying the site, and I don't have an account to do that with.
If you can leave a desktop computer running and connected to the network, you could probably access and run the site using a remote desktop application like Splashtop.
Hold in mind, because you are working with sensitive customer data you'll want to make sure no one can access your desktop (and watch) while you are using it this way. Some apps may blank the remote desktop screen and lock it when accessed, Slashtop does not.