Please do not change any plugin code. It will be overwritten when you update plugin.
There are lots of hook in this plugin. So you can change almost anything without touching plugin code.
To achieve this functionality without changing plugin code, please add following code in your theme’s ( child theme’s if any) functions.php

add_filter( 'fep_filter_user_name', function( $name, $id ){
    $name = fep_get_userdata( $id, 'first_name', 'id' ) . ' ' . fep_get_userdata( $id, 'last_name', 'id' ) . ' ' . fep_get_userdata( $id, 'last_name_suffix', 'id' );
    return $name;
}, 10, 2 );