I think there is more to consider here than is first apparent. If the client could well afford to have their install upgraded then I would walk away. If the client was an OAP,who could not afford to upgrade,and needed an extra socket...and in my opinion based on 30yrs in the game(most of it as a full NIC approved)...adding one socket not on an RCD and with no bonding would not reduce the current safety of the install,then yes....I may well do it as a cash job with no cert....I dont think that is necessarily unprofessional,especially when the said install has been in perfectly satisfactory use for 30 or 40 years,and probably will be for another 20 or 30.
It is possible to assess the risk from lack of bonding you know....what is the risk of simultaneous contact between CP's and ECP's in relation to the whole install and the minor addition?....assess and decide whether or not to proceed.....some might say that simply walking away is unprofessional.