Here's a couple videos on how to test for a current drain....
http://www.youtube.com/watch?v=yFCT-...eature=related
http://www.youtube.com/watch?v=pjXt7...eature=related
In many cases, a current drain is caused by taillights or domelight/glove box light staying on or a bad cigarette lighter socket which people use to charge cell phones and can go bad over time or a poorly installed aftermarket radio,
If the battery light is staying on, he may have other wiring issues or a bad alternator ...even new rebuilt ones may be defective....it might be better to have a shop check it out.