Please refer to the following code snippet demonstrating how the comparer can be applied:

        public RadForm1()
        {
            InitializeComponent();
            this.radGridView1.EnableFiltering = true;
            this.radGridView1.ShowHeaderCellButtons = true;
            this.radGridView1.FilterPopupInitialized += radGridView1_FilterPopupInitialized; 
        }

        private void radGridView1_FilterPopupRequired(object sender, FilterPopupRequiredEventArgs e)
        {
            RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
            if (popup != null)
            { 
                popup.MenuTreeElement.TreeView.TreeViewElement.Comparer = new MyComparer(popup.MenuTreeElement.TreeView.TreeViewElement);
            }
        }

        class MyComparer : TreeNodeComparer
        {
            public MyComparer(RadTreeViewElement treeView)
                : base(treeView)
            {
            }
            public override int Compare(RadTreeNode x, RadTreeNode y)
            {
                if (this.TreeViewElement.SortOrder == SortOrder.Descending)
                {
                    return x.Text.CompareTo(y.Text);
                }
                return x.Text.CompareTo(y.Text) * -1;
            }
        }