As I understand it, California isn't keen on double edged knives. Berkeley has a three inch limit as well as an outright ban on fixed blades.
EDIT: as well I think they tried to get him on the "waist area" requirement in the state law. He had it down on his calf.
There is nothing in the California penal code that prohibits the sale, purchase, possession, or open carry of double-edged fixed-blades, or the carry of double-edged folding knives, either openly or concealed. In fact, there is no mention of "double-edged knives" or "knives with two edges", etc, anywhere in the CA penal code.
If Clang is correct (and I will assume that he is) I can understand why your friend was arrested. Even if a knife is a fraction of an inch longer than the specified legal limit, that is an arrestable and prosecutable offense. I'd say your friend was very lucky to get away with just a night in jail.
As far as where a fixed-blade is openly carried on ones person, I don't know what Berkeley law says about it, but CA law does not require that an openly carried fixed-blade be carried on the waist. California penal code 20200 mentions that a fixed-blade worn openly in a sheath hanging from the waist is not considered "concealed" under the law, but that doesn't mean a fixed-blade must be carried in such a manner in order to be legal (so says my attorney). Many people (including myself in the past) carry fixed-blades in pocket sheaths, and some carry fixed-blades attached to backpacks or sticking out of the pockets of backpacks, all perfectly legal. In my first post in this thread I told the story of a cop who saw me carrying a fixed-blade in a back pocket, no arrest was made, and no warning was given about how I was carrying it.
Of course it's always possible for a cop to misinterpret CA pc 20200 and assume that a fixed-blade MUST be carried on the waist. Even cops make mistakes.