OK, time to clear up a few misconceptions here. In North America, your 120 volt general duty circuits normally consist of three wires - Line (which is hot), Neutral, and Ground. Neutral is connected to Ground at the breaker box. Older homes only ran the Line and Neutral wires to plugs and sockets, not a problem normally except for potentially disastrous safety consequences if you crossed the Line and Neutral when rewiring something and the fact that Neutral could go hot for long wiring runs. In fact he may even have one of these ancient knob and tube wiring configurations:
https://en.wikipedia.org/wiki/Knob-and-tube_wiring
Now to get to the dimmer switch problem. For some very good basic engineering reasons that I won't go into here, at minimum brightness there is a very small voltage difference, 17 volts RMS comes to mind for some reason, left between Line and Neutral, at maximum brightness it is 120 for an ungrounded dimmer switch. Normally this minimal voltage is shunted to Ground at the switch. He doesn't have a Ground wire, so putting in a single incandescent bulb supplies this shunt to turn the triac in the dimmer off.
So now we need some potential solutions. The first and most obvious is to rewire his house. Expensive proposition but would dramatically increase its resale value and also a lot safer which would reduce his homeowner's insurance. A much simpler proposition would be to replace the dimmer switch with a push type dimmer. On/Off is switched mechanically by pushing the knob, turn the knob to control brightness.
Personal note - the oldest part of my house was originally knob and tube, I made sure it was replaced before I purchased it.
edit to add - in my career I worked on some projects which had scenarios very similar to the above where we were using electronic switching (think dimmers on a larger scale) for devices. Along with that I spent a number of years as an electrical safety guy making sure that manufacturers had a clear understanding of when and where to put fuzes and insulation on their machines. A lot of stuff that is done in the UK/EU is forbidden in the US - think multiple breaks in the Neutral, fuzes on the Neutral side, etc.