It has been several years since last dealing with a case of tort so you'll need to check what I've said.
The studio owes a duty of care in providing a safe workplace for all employees and others on the premises. So assuming the studio was under a duty of care for the person then it would be responsible considering statutory health and safety laws. The studio in turn could then sue the camera fitter only if he is working as an individual. If he is working for a company the studio will seek damages form the fitter's employing company (provided he was fitted the camera whilst under a duty of employment).
If a duty of care is not established between the studio (there a numerous cases where an employer can do this given certain conditions)
Regarding causation and foreseeablity, if done correctly, one could argue or at least entertain that there was a direct link between the negligence of the camera fitter which caused death. There was no break in the chain of causation if the studio could reasonably rely on the experience and qualifications of the fitter, as to make the event which occurred unforeseeable on part of the studio.
Regarding the mother it is all down to circumstance. How bad was the shock-disorder, did it stop her from doing things normally?
Did either party owe her a duty of care?
This is all rather superficial but hope it helps