To reproduce: use the following code snippet:

public Form1()
{
    InitializeComponent();
    List<Item> items = new List<Item>();
    for (int i = 0; i < 10; i++)
    {
        items.Add(new Item(i,"Item" + i));
    }
    this.radListView1.DataSource = items;
    this.radListView1.AllowEdit = true;
    this.radListView1.ViewType = Telerik.WinControls.UI.ListViewType.DetailsView;

    this.radListView1.ItemValidating += radListView1_ItemValidating;
}

public class Item
{
    public double Id { get; set; }

    public string Name { get; set; }

    public Item(double id, string name)
    {
        this.Id = id;
        this.Name = name;
    }
}

If you try to edit "Item5" cell and press Enter key to confirm the value, the ListViewItemValidatingEventArgs.OldValue property in the ItemValidating does not show "Item5" which actually the old editor value.

Workaround: Currently, the possible solution  is to get the old value from  the ListViewItemValidatingEventArgs.Item[e.ListViewElement.CurrentColumn]: