Yeah, its kinda confusing the way they phrase it, but "He aha 'o Kēhau" is a way to ask "what is Kehau's job?"
But just like how you shouldnt use "What are you" in an English conversation, I dont think you should use it in a Hawaiian conversation either. I would use "He aha kou hana" instead.
This could also be a question asking about the gentle wind / breeze called Kēhau that West Hawai'i is famous for. It depends on the context and how you look at it.
"Pay attention to the accents," I was scolded. But I don't know how to do a kaha ko in Firefox if they don't pop up the little special keyboard. It's too bad the first romanizers of Hawaiian didn't treat long vowels as double. "Keehau" would be easy to type and to pronounce.