What I can suggest for achieving your re`uirement is handling selectionChanged event of the igCombo editor provider. Every time the selection is changed edit mode can be ended using endEdit method (with autoCommit option set to true the newly selected value will be committed) and afterwards entered again conditionally for the column of your choice. When trying to enter edit mode, using startEdit method this will trigger  the editCellStarting event. There you can check the value of the column with the combo editor and based on this value decide whether it will be read only or editable. Ending edit mode is necessary because when row is in already edit mode calling startEdit will cause an exception.

For example:

                    {
                        name: "Updating",
						columnSettings: [
							 {
								columnKey : "City",
								editorType: "combo",
								editorOptions: {
									dataSource: cities,
									textKey: "name",
									valueKey: "name",
									selectionChanged: function(evt, ui){
									var rowID = parseInt( ui.owner.element.closest("tr").attr("data-id"));
												$("#grid").igGridUpdating("endEdit", true);
												$("#grid").igGridUpdating("startEdit", rowID, "FirstName", true); 
									}
								}
							}
						],
					
						editCellStarting: function(evt, ui){
							cellToCompareVal = ui.owner.grid.getCellValue(ui.rowID, "City");
							if(ui.columnKey == "FirstName" && cellToCompareVal !== "Sofia" ) {
								return false;
							}
						}
                    },

Attached you will find a sample illustrating my suggestion for your reference. The "City" column has a combo editor with two values - "Sofia" and "Plovdiv". If "Sofia is selected the "First Name" column is going to be editable. In all other cases this column is going to be read only. 

Please test my sample on your side and let me know if you have any additional questions regarding this matter.

4863.igGridConditionalEditing.zip