That customer is dumb. It's not making them money when it's off. I'd have one simple conversation with the customer/owner stating the obvious and express how you want this handled going forward.
1.) Machine is to always remain powered on unless smoke is seen.
2.) You are to immediately be notified by phone if someone reports a problem. Leaving a business card taped to the back glass. They are to put an out of order sign on the machine (leave on top of back box).
3.) Let them know any further incidents will leave you no choice but to pull your services.

While this is nothing short of the "normal" expectation by an OP, some people are just plain dumb and need things laid out in black and white. I'd also recommend a service log is kept in the machine. If your service is ever questioned you have a log file to prove your innocence.