The FilteringControl is a Popup and it is shown on the screen over RadGridView control. GridViewColumns do not expose a method to open the filtering control from code behind instead of waiting for a user interaction.
You can still find the instance of the specific Popup and try implementing any modifications yourself.
sender, RoutedEventArgs e)
var popUp = headerCell.ChildrenOfType<Popup>().FirstOrDefault();
// do any modification on the Popup itself
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