Jeff,
Since the GridListControl that is displayed in the dropdown is generated every time the cell becomes active, you would need to handle this task dynamically.
One way to do this is to use teh CurrentCellShowingDropDown event, and hide the column immediately before it is dropped. Here is code that would do this. You might want check explicitly row or column index values (cc.RowIndex or cc.ColIndex) to make sure you are on the proper cell.
Private Sub gridControl1_CurrentCellShowingDropDown(sender As Object, e As Syncfusion.Windows.Forms.Grid.GridCurrentCellShowingDropDownEventArgs)
Dim cc As GridCurrentCell = Me.gridControl1.CurrentCell
If TypeOf cc.Renderer Is GridDropDownGridListControlCellRenderer Then
Dim grid As GridControl = CType(cc.Renderer, GridDropDownGridListControlCellRenderer).ListControlPart.Grid
grid.Model.Cols.Hidden(1) = True
End If
End Sub 'gridControl1_CurrentCellShowingDropDown