Try the following approach in RadToolTipManager's OnAjaxUpdate event:
|((Page.FindControl(args.TargetControlID) as ImageButton).NamingContainer as GridDataItem).OwnerTableView.DataKeyValues(0);
This is, try to reference the ImageButton from inside the event. Once you reference the button, its NamingContainer is a reference to the parent GridDataItem from which you can access the OwnerTableView and its DataKeyValues.
the Telerik team