I’d say if they knew he was nuts and funded the gun purchases then yes they are liable.
I don't see anywhere where they purchased the gun for him, but you raise an interesting point: If it was known that he was mentally unstable and likely to hurt someone (his therapist likely knew and if CA is like other states that would have to be reported to the State) then doesn't that just increase the State of California's liability? Or perhaps that of the Therapist?
Seems to me that would further insulate the parents --- but I'm no lawyer and don't know California law. Just trying to use some common sense and reason (which again, may not apply in the land of fruits and flakes known as California.)