alternator is fine. The alternator keeps the battery charged and the battery runs all the vehicle electrics (including the fuel injection). With a bad alternator, the first symptom is a dead battery.

Once you replace the battery with a new one, you think life is good until a few days later (or the same day if driving at night) the car goes dead (sometimes with you in it). Then your alternator is hosed.

If he has a gauge or (uses a multimeter) the alternator is easy to check. When he starts the car and it's running, the voltage potential across the battery will be higher (it's charging). He should be around 11.5~12 off, and 12.5+ if the alternator is working fine.

Inconsistent starting is most likely a starter solenoid or a bad electrical connection.