My friend has recently moved into a one bedroom apartment in California, but cannot locate the breaker box. Where should it be?

Could be anywhere, but it should be accessible to them and not in another tenant's space.

Curious, why are you asking on an internet message board for your friend? He/she should just call the landlord.

I have never lived in a rental in a multi unit building that had a breaker box in the unit.

Not sure, but if you can find the electrical meter, you can bet it will be near there. It could be outside next to it, or on an inside wall near it.

I have never lived in a rental in a multi unit building that had a breaker box in the unit.You couldn't have lived in many then. ;)

I've never lived in a modern building.
I lived in a 100 year old building in Harvard Square that had fuse boxes off the lobby, one 20 amp fuse per apartment. The fuse moved out towards you when you opened the box, and the power cut just by opening it. I ran everything - fridge, microwave, small low amp AC, computer, TV and lights on that 20 amps by being careful. The stove was gas. It was a 60 unit building and the city allowed it.

Thanks to everyone! The breaker box was located outside the building near the main entrance to her apartment. To answer stanfortyman, I used this media to find an answer because the landlord was away for the weekend and my friend needed her electricity. Thanks again, everyone.