Normal standard 12/2 romex is already EMF resistant, as the magnetic fields created by the two wires cancel out almost completely- the only reason it's not 100% is because the wires are separated by a few mm. You have to get right up on it to measure any field; at larger distances, they cancel pretty good.
If they're concerned about this, treat them like audiophiles and profit from their paranoia. A sealed metal raceway is an effective faraday cage. Wire the house entirely in compression-coupled EMT, and ensure every circuit is fitted with an EMF imbalance device (put a sticker on a GFCI). Make sure all the walls are covered with conductive foil. Caution them against using wifi or cell phones. Not that they'll will work with all the foil in their walls, but that's their choice.
If they're REALLY paranoid, you can built the entire home to HEMP standards, which not only protects them from nuclear EMP, but also from harmful emissions from their neighbor's wifi and their other neighbor's poorly sparking truck.
