I used to think he was just paranoid...
His mistrust might be about the phone's owner and the recording apps they might use, more than the smartphone itself.
Socially it's much safer to direct suspicion at "the smartphone" rather than the people invited to your house.
In reality, deliberately taking a second recording device other than your phone, escalates intention and risk. If we assume the visitors were unaware of the phone rule before arrival, there's no chance anyone will bring a second recording device.
> A visitor who wants to record...
Will use the recording device they already carry.
If discovered, your phone can hide behind a million excuses. "whoops, I forgot to end my call" or "whoops, I forgot to shut down my streaming app." It can also hide its recording status when using certain covert or custom apps. It is the ultimate recording device able to operate in the open.
Don't doubt it for even a second.
Not as easy as one may think. If you put a phone in a generic waterproof metal box (metal junction or biscuit box for instance) and call it with another one, most of the time it will still ring...
So your test may seem successful, but the box not be enough closer to a tower, or if your neighbor added a new AP, or if cell phone frequencies changed since you bought and tested it. Moreover, if you have to open and close it often, you have to be careful that the attenuation doesn't degrade.
All that to say that the "Faraday box solution" would mean testing it regularly with professional instruments and all that. It is far simpler and cheaper to ask guests to leave their phones in their cars.