I would ABSOLUTELY have her sign a "hold harmless" agreement.
that's not going to do a bit of good you can't have someone agree to let you break the law .
Audio and video monitoring of employees is perfectly legal
Ive done some research on it for a job and its not a black and white issue
I suggest you talk to a attorney as to CA law and your liability to protect yourself or pass on the job
that said , a local CATV supplier here has mics and stickers you put on all entrances to the bldg informing anyone who enters that audio is being recorded . I just got a sale flyer from them and one of the items is a hidden camera with audio built into a non functioning motion detector for 40
they also sell the mics and can help you set it up
PM sent
the main national CATV supplier wont sell the stuff because of the legal ramifications