private void RadTreeListView_Filtering(object sender, Telerik.Windows.Controls.GridView.GridViewFilteringEventArgs e)
{
    var treeListView = sender as RadTreeListView;
    var internalColumns = treeListView.GetType().GetProperty("InternalColumns", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).GetValue(treeListView);
    internalColumns.GetType().GetProperty("ColumnWidthsCalculationPending", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).SetValue(internalColumns, true);
}

private void RadTreeListView_Filtered(object sender, Telerik.Windows.Controls.GridView.GridViewFilteredEventArgs e)
{
    Dispatcher.BeginInvoke(new Action(() =>
    {
        var treeListView = sender as RadTreeListView;
        var internalColumns = treeListView.GetType().GetProperty("InternalColumns", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).GetValue(treeListView);
        internalColumns.GetType().GetProperty("ColumnWidthsCalculationPending", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).SetValue(internalColumns, false);
    }), System.Windows.Threading.DispatcherPriority.Loaded);
}