To do what you described, I would suggest using TableControlCurrentCellValidating. This is hit once as your user leaves the cell. This code will then copy teh value in ParentName to the ParentDec column.
private void gridGroupingControl1_TableControlCurrentCellValidating(object sender, GridTableControlCancelEventArgs e)
{
GridCurrentCell cc = e.TableControl.CurrentCell;
GridTableCellStyleInfo style = e.TableControl.Model[cc.RowIndex, cc.ColIndex] as GridTableCellStyleInfo;
if (style.TableCellIdentity.Column != null &&
style.TableCellIdentity.Column.MappingName == "ParentName" &&
(style.TableCellIdentity.TableCellType == GridTableCellType.AlternateRecordFieldCell ||
style.TableCellIdentity.TableCellType == GridTableCellType.RecordFieldCell))
{
string newvalue = cc.Renderer.ControlText;
GridRecordRow rr = style.TableCellIdentity.DisplayElement as GridRecordRow;
rr.ParentRecord.SetValue("ParentDec", newvalue);
}
}