by the sound of it, I'm not sure if there is a worse job!
If my manager was doing this, then I'd most certainly quit. Can't your friend contact head office, or her area manager and tell them what's going on? Half the time head office have no idea what their employees are up to.
On recently quitting my job, I was given an exit interview, where I had to state why I was leaving, was I happy with the staff/manager/store etc etc.. this helps the make the company a better place, and, sort out anything dodgy going on. I'm not sure if all businesses do this now but it would be a good way of letting the employers know as it's all done online.
In my opinion, it is unfair and highly unprofessional to use sound recording devices in the store, purely with the intention of invading staff's privacy, which is what it sounds like. I'm sure if the customers knew that this was the case they would cease to purchase food from the shop/cafe again.
Hope this helps? Lou x