Oh dear, two degrees, but blissfully unaware of the responsibilities and liabilities. I too have a degree in Electrical and electronics as well as being a Chartered Engineer. I also was involved with the last IEEE regs (not the latest) and ran at one stage a company that designed and manufactured electrical test instruments. I can play top trumps too
There isn’t a contract that exists that allows the customer irrespective of whether the work paid for or free to sign away the fact that someone may create a lethal or dangerous bit of kit. If you fix it, you are personally liable for your errors as well as the organisation that organised for you to do it.
Knock yourself out, if you feel like playing Russian Roulette. But from what you say about how these cafes operate they should be shut down immediately, extremely ridiculous way of operating.