Legal Question in Civil Litigation in California

You need to have someone who works with those documents so can testify that they are in fact the actual billing records [custodian of the documents].

It needs to be the custodian of records. It does not have to be an accountant. An accountant is necessary only to provide expert opinion testimony regarding what the records prove or don't prove, but is not necessary for authentication.

To simplify the previous correct answers, anyone at the company who can testify from their personal knowledge that those are the company's actual billing records can testify to have them introduced as evidence. To prove the truth of the contents of the records, however, you will probably need someone who knows enough about the billing practices to authenticate the contents as well as the documents themselves.