Pretend to use a cellphone in her presence. When she starts complaining of symptoms and discomfort, show her that the phone not only isn't on, that it doesn't even have a battery in it so there's no chance it could have been on.
I did something similar to this with a friend of mine who claimed to be able to see infrared light from TV remotes. While he wasn't looking I removed the batteries from one, then called his name and when he turned around, pointed it at him and pushed buttons. He complained about how much that hurt his eyes, and how could I do that to him? Then I showed him the remote had no batteries in it. Needless to say he was somewhat embarassed. Still claims to be able to 'see' IR light though.
Well, I'm apt to believe this lady more than the other case. Why? Because EMF sensitivity is actually real, However, it's generally NOT frequency specific, so you pretty much have to isolate yourself.
There's a guy in Finland who worked for Ericcson who suffered from it, and he got a special EMF suit that worked for a couple of years. He lives out in the countryside far from civilization - including cellphones, power lines, etc.
This lady does the same - she's living far from civilization with no electricity, which lends credence to her symptoms.
However, the problem is, this is two cases among millions who claim the condition. You can easily tell who's BS'ing the condition because they're among regular people - if you have this condition, you have to move to basically pre-civilization. You can't have a car, no cellphones, no electricity, nothing. You can use gas appliances like a stove, but you can't use a motor (with electric spark based ignition).
So what's the easiest way to tell? If they're not isolating themselves in far off places with basically no one around, then it's likely real. If they're still walking in normal civilized areas, it's fake. Sorry, if you have this condition, self-isolation is the only way to get relief. You cannot claim to be "sick of WiFi" when you're walking down the street in any city or town - if you really had the condition, the EMF from *power lines* will do you in. And forget about using technology.
So maybe this lady has the problem. But you know who doesn't? Everyone else. Those who live in the "quiet zone" certainly aren't affected by EMF (remember, power lines also affect them, ditto with cars and other technology). The lady saying WIFi made her kid sick isn't real either - if her kid was really sick from EMF, she'd move to the middle of nowhere.
It's a real condition, but you'll know if the person is lying or not just by how they act. If they're out miles from civilization, no technology, no electricity, no cars or other ICE style locomotion (steam, and old-style diesel engines are fine), then it's likely real. If you're mingling in normal civilization with technology, cellphones, internet, cars, computers, TV, radio, tablets, etc., it's fake. The affliction is pretty broadband going down all the way to practically DC,
Oh yeah, if you want visitors, they have to park their vehicles about a mile away, so you'll want to provide them with a bicycle or other human-powered conveyance. (And no, you can't visit them, because the EMF will be unbearable once you get close).
With that criteria, you can tell Chuck from Better Call Saul is definitely faking it.