the few times I've seen the alt go bad it completely drains the batt. he wouldnt have any power to the radio or windows. Sounds to like the batt has just enough power to turn on the acc, but not enough to take the load from starter. the best way to tell is to jump the car and see how much power the alt is puttin out, should be around 14 volt. also a voltage drop test on the batt to confirm that the batt if the cause.