In order to investigate this problem we would need a bit more of your implementation. Could you please share how you are retrieving the appointment context menu object on the client? For example there is already a variable that holds a reference to the appointment context menu client-side object.
RadScheduler1_AppointmentContextMenu( sender, args )
SelectedAppointment = args.get_appointment();
AppointmentContextMenu.show( args.get_domEvent() );
Could you please specify whether that RadScheduler1_AppointmentContextMenu function is a handler for the RadScheduler OnClientAppointmentContextMenu or the OnClientAppointmentContextMenuItemClicked event?
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.