It may not be that simple. One thing that makes a humbucking pickup work as well as it does is that the two coils are crammed in next to each other so they "see" almost the same external field. When you separate them they don't cancel the noise quite as well. Also, a short piece of wire is not that susceptible to picking up RF. I'm not saying not to shield his bass; if the shielding is lacking then he definitely should make it better, irrespective of where this particular noise is getting into his signal. He should also make sure that the bridge and strings are solidly grounded to the shield in his cable. If one of his pickups is not RWRP, then he should consider changing one of them to RWRP.