12 kw is 50 amps at 240 volts so that is the rating of the generator and you can connect 80% of that. If he wants to run the whole house he needs a bigger generator. So what is the solution? Take the most essential loads like the fridge, some lights, any pumps essential to sanitation or flood protection possibly. Add a bit of heat for one room and a few convenience outlets for the wireless phone and a hot plate and you are getting to the limit. The load center must be connected through an approved transfer switch with a source from the regular panel and 1 from the generator. Both sources would have a 50 amp breaker ahead and feed a 60 or 100 amp loadcenter through the transfer switch. The grounding at the generator must be checked to ensure the neutral is not grounded as most transfer switches will have a continuous neutral.
The problem is if you want to have all lights in house and possibillity of using all plugs,in this case you need many circuits to put on subpanel(plus fridge,subpump,furnace etc). The easiest way is to install main disconnecter after meter and transfer switch (service rated) and connect generator. Customer knows that generator is not big enough for all house(full calculated load) and will take control of load. Generator is protected of overload so that I can't see a problem for this solution. If customer is not at home in case of blackout generator is big enough for load remain switched on.
Mj it is still a violation to connect a load greater than the generator capacity so the essential circuits must be seperated from the main service and fed via the transfer switch through a load center to 80% of the max rating of the generator.