he needs to have the battery or the alternator tested or both b/c:
it still could be the battery. the battery is used under high electrical load conditions like with with your stereo on and blasting and maybe other stuff going. Plus if you're at idle for an extended period during rush hour, and the dying battery is not helping with the high load, you're going to have power problems, maybe indicated by a car that is about to stall or does stall. My car did stall under these conditions: bad battery, stereo on and a/c on, and at idle stuck in traffic. I replaced the battery and have had no problems since.
is the AEM
pulley underdriven? does it come with a belt?
From a mustang webpage on pullies:
Commonly Expressed Concerns about Underdrive Pulleys:
1. Lowering the alternator speed will cause the battery to run down. That may be true for some people. If your driving style includes idling the car for long periods, or if you have a high end audio system that requires constant power from the alternator, underdrive pulleys may not be for you. The hot idle speed for a 2000 Mustang 3.8L V6 engine is 700-800 RPM. ASP claims that with its 25% underdrive set the alternator will provide a full charge when the engine is turning over at 850 RPM. I don't expect any problems in this area.
so it might be a bad battery + the alternator underdrive pulley + stereo + idle,
OR it might just be underdrive alternator pulley + idle + stereo.