Can a subpoena be served by mail in CA?

My daughter's friend's mother has told her she has sent a subpoena and must be in court today. She has not received the subpoena at school, just been told. She doesn't want to get involved in a disputer for $$$ between her parents. Her mother claims she was forced to sign agreements under duress that would preclude her mother from receiving any $$ in the case of a divorce. Apparently the mom and daughters have been victims of violence by the father throughout their lives.