I'll recommend you to use so named Inline Editing for row editing. The most advantage of this method, that it is very intuitive and the user. You can see how it works on the demo page http://trirand.com/blog/jqgrid/jqgrid.html. Choose on this demo Row Editing and then Using Events or Input types on the left tree part. With this method you can implement any custom verification whether the selected row should be allowed to be edited or not inside of the event handle onSelectRow or ondblClickRow. If you allow editing, then you call editRow method of jqGrid. This method creates input controls for all editable columns and the user can modify the row values in a natural way. The modifications will be saved if the user press enter key or canceled on esc key.

I personally prefer to implement calling of editRow method inside of ondblClickRow event handler. So the user can continue selecting of rows like usual and can use double click for the row editing. The pseudo code will look like folowing:

var lastSel = -1;

var isRowEditable = function (id) {

    // implement your criteria here 

    return true;

};

var grid = jQuery('#list').jqGrid({

    // ...

    ondblClickRow: function(id, ri, ci) {

        if (isRowEditable(id)) {

            // edit the row and save it on press enter key

            grid.jqGrid('editRow',id,true);

        }

    },

    onSelectRow: function(id) {

        if (id && id !== lastSel) {

            // cancel editing of the previous selected row if it was in editing state.

            // jqGrid hold intern savedRow array inside of jqGrid object,

            // so it is safe to call restoreRow method with any id parameter

            // if jqGrid not in editing state

            grid.jqGrid('restoreRow',lastSel);

            lastSel = id;

        }

    },

    pager: '#pager'

}).jqGrid('navGrid','#pager',{edit:false});