fhqwhgads

I don't know how it works in California, but in my home state an interested party can go to the district courthouse and file paperwork called an "emergeny petition". Basically you explain why you think your friend may be a harm to himself or to someone else. You then need to swear an affidavit in front of a judge stating that the information that you wrote in the document is true. The police then get a copy of this paperwork, pick your friend up, and take him the the local hospital where he can speak to a mental health professional. Based on that interview, your friend is either released or held for further treatment/testing.

Start by calling the clerk's office at your district courthouse. They may be able to get you started....

Best of luck to you and your friend.

edit: If you feel an urgency, call the police. However, in my state, the police actually have to have first hand knowledge that your friend is a danger. (They can't just take your word for it)