If all else fails he can add a relay with a 24VAC coil to the circuit. The
button would operate the old chime directly and also feed the coil of the
relay. The contacts of the relay become the the "button" to the new chime
and whatever need to be done with the diode (across the contacts
apparently) is done at that point. One country--two systems! It'll be the
Hong Kong of doorbells!