It sounds like your friends device is not going into a "deep sleep", a suspended state which allows the phone to use minimal battery. Usually it's an app that keeps some amount of processing active enough to prevent your phone from going to sleep. For folks that heavily use their device this usually goes unoticed since the battery drain is not any faster under constant usage, for the casual user the battery life hit is huge.
I can't tell you what might be causing the problem as I don't know what your friend has installed nor do I own a Galaxy.
This issue was happening to my device when I updated my ROM from stock to an ICS rom based on Android 4.0.3. It took me a while to track down the problem unfortunately the problem was the GPS. Once the GPS was used the phone would not go to sleep. I would get great battery life so long as I did not use the GPS. So the work around for me was to disable GPS and reboot. Not an option so I usually keep my phone charged.
My recommendation is for your friend to restore his device back to factory settings, note this will remove all of his contacts and applications. He can then use Google to restore his contact list and even apps, if he has setup his google account this way. Personally I would recommend he restores his contacts via google and selectively install his apps as restoring both will be hard to find out which app is preventing his phone from sleeping.