The "400 Session Timed out" page is a response page from server. This page can be modified to show the the login screen or can be auto redirected to login page after some time out value when that page appears.
But all these things should be taken care by server application. Enterprise browser have no control over what server responds and Enterprise Browser is not designed to manipulate the server side response.
So please advice the customer to work on the SAP page to show login screen when session is timed out.