Alternator? 1997 Mustang V6

The battery light will come on and also show low voltage. Now this doesn't happen all the time. It can be driven for 40 minutes and be fine, then the light will come on as well as the low voltage. BTW, this isn't my car just trying to help a friend. The local autozone checked the charge while running and it was fine. Of course the light wasn't on at that time. What would cause this? Thanks all! Mike