The customer followed the setup instructions for setting up "Persistent Connections" in his environment as described in our documentation under:
Enabling persistent connection in your environment
However, when he goes to his Site Server, the option for "Persistent connection is disabled" is grayed out and he can't edit it.
Misconfiguration. As stated in our documentation under Step 3 in Enabling persistent connection in your environment
"If the Configure HTTPS binding option is unchecked and the HTTPS port is not specified, the WebSocket server does not start on the Site Server and the persistent connection will not be available."
The customer didn't have the box checked for "Configure HTTPS on Site Servers".