Without GNU, that's just a fever dream.
Many cancers are survivable these days. I hope he has one of those.
You get an ESP8266 micro with wifi plus a power supply, relay, momentary button, current and voltage sense, and a couple LEDs all for about $8. Serial debug and flash headers are broken out for easy access on the PCB.
They ship with chinese firmware but the headers and standard hardware make them dead simple to flash with your own firmware, or ESPhome or Tasmota if you prefer.
Shelly devices offer a firmware that can be controlled trough a REST API locally. Unfortunately it's still proprietary and not open, but it doesn't require a cloud connection.
Otherwise you can buy a device and replace the firmware, there are number of open alternatives, such as ESP Home, Tasmota, etc.
Or... you can build it yourself. Building a smart plug is an easy task, if you have some practice on electrics. You will likely build a better product in terms of safety and capabilities that one you can buy.
The primary reason off the shelve products are cloud etc. is because these companies spent the time and money to do the above and since no on wants to pay 100+ for an iot switch they add cloud garbage etc. These products are now sold to the masses and if you have to support them you need control over them or your costs go through the roof.
I am working on a hardware iot product (no cloud) and I have to tape off the USB service port not because there is anything that could go wrong but because people don't read instructions and think the thing will power over USB when there is a power supply included with a barrel plug...
Interestingly, even after years of professional hardware engineering, I still feel some hesitation when interfacing with 230V AC.