To fix the issue immediately, ask the user to switch to HTML viewer and he should not be seeing the error.
Go to BI launchpad->Preferences->Web-Intelligence->choose HTML for viewing. Restart the browser and he should be able to open the report without any errors.
Regarding the java error, you need to make sure that the user has the right version of java installed. You can refer the guides or simply, check the java version on a PC where you are able to open the report without any issues.
Regards,
Naras