Set field required based on another field value using events
An example is to set mandatory fields based on another field value. The solution is use add_event() method and 'jqGridLoadComplete' event. See this example:
http://phpgrid.com/example/row-level-permission-edit-condtion/
The javascript required to set column property to be required
// ... your condition ...//
jQuery(”#MYGRID”).setColProp('colname',{editrules:{required:true}});
More about jqGrid setColProp method:
http://www.trirand.com/jqgridwiki/doku.php?id=wiki:methods