A possible solution is to disable the grouping when export to Excel command is fired. This way the grouping will be removed only the export file and will not effect the rendered RadGrid. If you are using the built-in buttons then you can hook OnItemCommand event handler and disable the grouping as demonstrated below.
sender, Telerik.Web.UI.GridCommandEventArgs e)
(e.CommandName == RadGrid.ExportToExcelCommandName)
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.