Well I'm not sure if it works in all distros, but in Ubuntu, you must be root to change the permissions of a file. If you have permissions for being so, hit Alt+F2, type "gksu nautilus" and the path of the containing folder. Then you'll be able to edit the file. You can do it by terminal as well. If it's a text file, type "sudo gedit (path of the file)".
Surely you should talk to your friend before making any changes in his file, but if he can't edit it as well, it means you have to be root user to do so.