I am sorry to hear this.http://www.dca.ca.gov/publications/landlordbook/discrimination.shtml
"Unlawful housing discrimination can take a variety of forms. Under California's Fair Employment and Housing Act and Unruh Civil Rights
Act, it is unlawful for a landlord, managing agent, real estate broker, or salesperson to discriminate against any person because of the person's race, color, religion, sex (including pregnancy, childbirth or medical conditions related to them, as well as gender and perception of gender), sexual orientation, marital status, national origin, ancestry, familial status, source of income, disability, medical condition, or age in any of the following ways:
Refusing to make reasonable accommodations in rules, policies
, practices, or services when necessary to allow a person with a disability "equal opportunity to use and enjoy a dwelling" (for example, refusing to allow a person with a disability's companion or service dog)."
So as noted above, the dog is protected under the FEHA as well as the ADA, seehttp://www.ada.gov/service_animals_2010.htm
You can send that information to the landlord's agent, but it would have more impact if you get a local attorney to reply to the letter.
You can get a free consultation from some of the discrimination lawyers listed by location athttp://lawyers.findlaw.com/lawyer/practicestate/Discrimination/California
I hope this information is helpful.