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.

Microwaves degrade everyone's 2.4 GHz ISM band radios. Wi-Fi occupies the ISM band because it's a garbage band that was already wrecked by this type of equipment. There isn't a solution to this; the safe leakage from a microwave oven is millions of times more powerful than wifi field strength. 5 or 6GHz band avoids the problem.

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.

From the fine article, second paragraph:

"you can build a tiny “crystal detector” that responds to 2.4 GHz Wi-Fi, Bluetooth, and even microwave oven leakage"

If they're seriously concerned, you can get microwave leak detectors for very little money from most crapvendors. The problem with this design is that while it's a cool toy it'll be prone to false negatives...

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?

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

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