To reproduce:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.ProductsTableAdapter.Fill(Me.NwindDataSet.Products)
    Me.CategoriesTableAdapter.Fill(Me.NwindDataSet.Categories)

    RadGridView1.AutoGenerateHierarchy = True
    RadGridView1.DataSource = Me.NwindDataSet
    RadGridView1.DataMember = "Categories"

    Me.RadGridView1.MasterTemplate.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    Me.RadGridView1.MasterTemplate.Templates(0).AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

    Me.RadGridView1.UseScrollbarsInHierarchy = True
End Sub

Private Sub RadGridView1_ViewCellFormatting(sender As Object, e As CellFormattingEventArgs) Handles RadGridView1.ViewCellFormatting
    If e.Row.HierarchyLevel > 0 Then
        e.CellElement.TableElement.RowHeaderColumnWidth = 100
    End If
End Sub