One of our lab technicians brought suit against the company in the mid '80s claiming that electric fields encountered in testing caused his leukemia diagnosed via our company yearly physicals.
We all had yearly physicals 'just in case' as an air force Colonel involved with EMP testing had been diagnosed with leukemia a few years earlier. Emf and power lines and illness were first being touted at about that time. Think the employee is still alive, as the diagnosis was very early. He had been working the area less than 3 years, one of the doctor's data points was than there were no recorded cases of anyone ever having been diagnosed with leukemia in less than 3 years after know exposure to even IONIZING radiation.
Saw lots of evidence presented in the course of pre-trial (settled out of court just before trial) that there was no emi effect on humans at less than UV frequencies except for heating at high field strengths (10's of kV per meter).
Personally have been in 2-3 kV/meter electric fields at up to 100 Mhz and could never physically/biologically detect anything (and were trying to detect any sense organ signals - no smell, no taste, no hearing, no visualy blurring, no palpitations) , household fields are in the few single digit volts/meter even next to induction cooktop. Welders are continually in high magnetic fields.
Just a few hundred volts/meter electric field will make any unshielded CRT go totally fuzzy, or create negative time sweeps on a scope.
tidbits:
- homing pigeons are known to lose there way near dc power lines (or magnesite deposits), indicating a capability of detecting electric and magnetic fields
- one co-worker who lives adjacent to 345kV line can get 1/8" sparks across a gap in his galvanized gutters
- per wiki, The majority of provocation blind trials to date have found that self-described sufferers of electromagnetic hypersensitivity are unable to distinguish between exposure to real and fake electromagnetic fields- personal experience that if a welding cable is within a foot of a CFL lamp it will fry the CFL (tore one down and found the failure mode was magnetic saturation of one leg of the tiny inverter transformer)
- have one grandson that started complaining in our house about a stabbing noise just after I'd installed some new electronic ballasts; tested grandson's hearing and found he could hear up to 23 kHz!, so the 20 kHz ballast switching frequency was audible to him, but no one else.
That all said, the previous comments about dealing with the customer and the customers perceptions (does not make any difference if it is real or not) are the main points to consider for the OP's situation, nothing else matters, e.g don't bother me with facts. Twisted wires in steel (not pvc) conduit are a good first step if the customer can be made to believe any physics facts - eg. a sensitive field strength meter on a NM cable vs next to conduit. Follow whatever techniques the customer proposes (and is willing to pay for) if they make sense or not, it is always the customer perception is 'correct' vs. physics. As an alternate, one could propose an Amish lifestyle?
Case history of sensitive lines: in the late '70's had to run a 300 ft power line and data lines near a Navy satcom station on Ohau. Site was going to have 50kV/meter EMP tests run, so put all the lines in rigid conduit - with all the couplings welded to assure continuity. No problems with the power or data lines.
BTW, FWIW, personally am getting a high power 130 ft cell tower installed in our OWN backyard.