This is a design website, not an accounting law website. You’d be best talking to an accountant.

In California, Is it illegal to do payroll before the payperiod is over?

In a word, no.

Can you help me find anything that shows that this isn’t right.

Right, or isn’t right depends on a lot of things in California. Is your friend actually an “employee” vs. a " independent contractor" (freelancer). Is your friend working for hourly wages? Is he working for a fixed salary (overtime exempt)? What did he agreed to when he took the job? What “industry” he is working in (wage laws affect film-industry employees differently for example), etc.

Payment of overtime wages earned in one payroll period must be paid no later than the payday for the next regular payroll period. Only payment of the overtime wages may be delayed until the next payday, not straight time wages. All employers in California are required to maintain payroll time records. All employees in California are entitled to view, and copy (or have copied), their payroll records within 21 days from date of request. You are entitled to an Itemized Wage Statement as well. BUT ALL OF THE ABOVE IS DEPENDENT UPON ONES EMPLOYEE STATUS.

Refer your friend to the California Department of Inustrial Relations; Paydays, Pay Periods, and Final Wages. See: Q&A: #8

The reason I always kept a personal diary of hours worked.