Try to handle the ShownEditor event as the following (semi-pseudo code):
var grid = sender as GridView;
if (grid.FocusedColumn.FieldName == "Value") {
var row = grid.GetRow(grid.FocusedRowHandle) as // your model;
// note that previous line should be different in case of for example a DataTable datasource
grid.ActiveEditor.Properties.ReadOnly = // your condition based on the current row object
}
This way you could refine the already opened editor with your needs.