On the type, the crew initially would enter the expected fuel mass manually and the FMS shows ZFW+expect.Fuel = AUW during cockpit preparation and loadsheet check (*). Until this point, your colleague's solution would work.
After the engines start, the FMS would erase the manually entered fuel weight and add the crew-ZFW to the actual amount of FOB (**). And the problem unfolds, there's nothing in your post I see amiss.
Too smart of a solution. Probably because your system does not allow to modify the non-standard fuel loading index.
(*) when checking the LDS, before engine start, the crew will verify the manually entered FOB value is the same as the actual fuel load. The difference would be identified as deliberate in your scenario.
(**) That can actually be overwritten but then you are really causing a serious headache.