My dad was driving his Sentra the other day, went into a store, came out tried to start it, and nothing. No warning, nothing. He got it jump started but it died a few miles down the road. It's in the shop right now. The battery is 6 months old, and the alternator was checked and was told it's fine.

No idiot lights come on either. When he drives down the road all light (headlights, turn signals are really dim). What could be the problem, since the alternator was checked and was said to be fine?