The first thing to try is a free program called |VLC. It's not specifically designed to recover corrupted video files, but it could be that the header is damaged and VLC will just ignore that. It's a great player that will play just about anything. Joe tried this and it was unable to play the file as well.
The Chatroom says to try HD Video Repair. Make sure to make a copy of the file first, so if it doesn't work, he won't make the problem worse.
This is another argument for backing up data, because then he would have another copy that might not be damaged.