Sounds like he’s in need of a new microwave. Not tongue-in-cheek; sounds like there’s something up with the shielding, and if it’s not visible, how will you know if it gets worse?

Edit: I see no reason this wouldn’t work, however?

I've tested a few microwaves from different manufacturers with my phone a few years ago. I think I looked at some file in my router (OpenWRT), but I can't recall. I got a lot of dropped packets each time. The amount of degradation was similar for the different microwaves.

I had to put the phone close to the microwave to detect this. The degradation was obviously stronger when the phone was closer.

If your friend experiences noticeable degradation regardless of the distance within the room, it might be worrisome.

But I think it's normal to have some interference. That doesn't necessarily mean enough of the 2.4 GHz radiation escapes the microwave to be harmful to an animal, as Wi-Fi, Bluetooth and so on are very weak, comparatively.

Funny thing is, after putting my phone inside a closed turned off microwave, it got Wi-Fi, although very weak. I didn't try that with all the microwaves, but with 2 or 3 of them.

I think the Faraday cage around the microwave was built to be good enough for safety, but it wasn't built with Wi-Fi interference in mind.

Disclaimer: I might be wrong, as I don't have enough background to make any bold claims.

For devices where connectivity is more important, switch them from 2.4Ghz to 5Ghz to avoid microwave interference.

The article says: "responds to [...] microwave oven leakage".

You could subsitute the LED for a moving coil meter: https://www.codrey.com/electrical/moving-coil-meter/

They have am much lower current and voltage requirement, so might me more sensitive. You can also do tuning to use different sized antenna. However that strays into analogue eletronics, which I've not really touched for 15 years.

Faster diode, like HSMS-2850 [1], and different leg lengths (tune for 5Ghz).

[1] https://datasheet4u.com/pdf-down/H/S/M/HSMS-2862_AgilentTech...

For 2.4 GHz each leg of the antenna should be 30 mm. For 5 GHz just under 15 mm.

See Dipole Calculator: https://www.omnicalculator.com/physics/dipole

And you need a faster diode.