My friend said once "this is very bad idea, to use another's wires", but this was more than 20 years ago, when this technology was extremely rare and expensive,

My friend said once "this is very bad idea, to use another's wires", but this was more than 20 years ago, when this technology was extremely rare and expensive, so where very small probability of such cases. Really just transformer with ferromagnetic core is enough to isolate such things. Now world changed, but there are lot of old tech devices without any security, designed decades ago.