Cochlear implants have an internal, implanted, component - the receiver and the electrodes - and an external component - the microphone, processor and transmitter. Typically, the external components are held on to the head by a static magnet and transmit to the implanted receiver using radiofrequencies.
In general, it appears that static magnets should be avoided - because they can interfere with the magnets used to attach the external receiver - and anything that injects currents in the head should be avoided because of the risk of damaging the implant. Static electricity was recognised a problem in the past but may be becoming less so. But sources of EMFs do not pose any risks, although they may produce a noise through the implant.