Can an employer withhold an employees tips?

The following Labor Code sections should answer your question:

350.  As used in this article, unless the context indicates
otherwise:
   (a) "Employer" means every person engaged in any business or
enterprise in this state that has one or more persons in service
under any appointment, contract of hire, or apprenticeship, express
or implied, oral or written, irrespective of whether the person is
the owner of the business or is operating on a concessionaire or
other basis.
   (b) "Employee" means every person, including aliens and minors,
rendering actual service in any business for an employer, whether
gratuitously or for wages or pay, whether the wages or pay are
measured by the standard of time, piece, task, commission, or other
method of calculation, and whether the service is rendered on a
commission, concessionaire, or other basis.
   (c) "Employing" includes hiring, or in any way contracting for,
the services of an employee.
   (d) "Agent" means every person other than the employer having the
authority to hire or discharge any employee or supervise, direct, or
control the acts of employees.
   (e) "Gratuity" includes any tip, gratuity, money, or part thereof
that has been paid or given to or left for an employee by a patron of
a business over and above the actual amount due the business for
services rendered or for goods, food, drink, or articles sold or
served to the patron. Any amounts paid directly by a patron to a
dancer employed by an employer subject to Industrial Welfare
Commission Order No. 5 or 10 shall be deemed a gratuity.
   (f) "Business" means any business establishment or enterprise,
regardless of where conducted.

351.  No employer or agent shall collect, take, or receive any
gratuity or a part thereof that is paid, given to, or left for an
employee by a patron, or deduct any amount from wages due an employee
on account of a gratuity, or require an employee to credit the
amount, or any part thereof, of a gratuity against and as a part of
the wages due the employee from the employer. Every gratuity is
hereby declared to be the sole property of the employee or employees
to whom it was paid, given, or left for. An employer that permits
patrons to pay gratuities by credit card shall pay the employees the
full amount of the gratuity that the patron indicated on the credit
card slip, without any deductions for any credit card payment
processing fees or costs that may be charged to the employer by the
credit card company. Payment of gratuities made by patrons using
credit cards shall be made to the employees not later than the next
regular payday following the date the patron authorized the credit
card payment.

353.  Every employer shall keep accurate records of all gratuities
received by him, whether received directly from the employee or
indirectly by means of deductions from the wages of the employee or
otherwise. Such records shall be open to inspection at all reasonable
hours by the department.

354.  Any employer who violates any provision of this article is
guilty of a misdemeanor, punishable by a fine not exceeding one
thousand dollars ($1,000) or by imprisonment for not exceeding 60
days, or both.

355.  The Department of Industrial Relations shall enforce the
provisions of this article. All fines collected under this article
shall be paid into the State treasury and credited to the general
fund.

356.  The Legislature expressly declares that the purpose of this
article is to prevent fraud upon the public in connection with the
practice of tipping and declares that this article is passed for a
public reason and can not be contravened by a private agreement. As a
part of the social public policy of this State, this article is
binding upon all departments of the State.

I would advise you to contact the Labor Commissioner's office.