public class MyTreeViewDragDropService : TreeViewDragDropService
{
    public MyTreeViewDragDropService(RadTreeViewElement owner)
        : base(owner)
    {
    }

    public void HideHintWindow()
    {
        this.HintWindow.Hide();
    }
}

private void RadTreeView1_DragOverNode(object sender, RadTreeViewDragCancelEventArgs e)
{
    ((MyTreeViewDragDropService)this.radTreeView1.TreeViewElement.DragDropService).HideHintWindow();
}