IMO there are lots of possible alternatives in between knowing everything or nothing. One would be that AK didn't know what was going on in Melissa's business affairs because he was not involved other than to live in the same house as her office. Even her investors didn't know what was going on, and that was despite getting reports once a month and even visiting Melissa's house to discuss their investments: "One victim, [FR], recalls that
whenever she and her wife [C]
met Caddick at her home to go over their finances, Caddick repeatedly licked her lips." -
The Age, 24 Apr 2021 (BBM)
I can easily understand that AK might not have had the capacity to understand financial affairs, because I don't have much of that capacity either (can anyone explain how Melissa was able to make money by forging her boss's signature on cheques? Would she have set up a sham company owned by herself and be making payments to that?). Maybe in referring to a lack of "capacity" his father meant a lack of intelligence, but I think it's more likely he meant that AK did not have the financial skills or knowledge or - more importantly in the circumstances - the
scepticism to recognise that Melissa was allegedly conning everybody.