When one checks in baggage at the airport, does the computer give the check-in agent the baggage allowance, or does he/she rely on memorized rules?

My parents will be flying in CX J JFK-BKK and CX J BKK-JNB. I know that the standard rule is that the allowance is 2 bags, 70 lbs. each for all flights, as the journey starts in the USA.

However, they'll be originating in Brazil, flying on JJ J GRU-JFK the day before, on the same ticket, causing the allowance to be determined at each departing city.

So with that, the BKK-JNB portion is actually only 30kgs!

Will my parents have any chance of getting away with checking 2 bags at 70 lbs. each with no excess baggage fees in BKK, claiming that they originated in the USA?

Should I get my parents prepared to hand over their excess baggage to my relatives at BKK before they leave for JNB and have my relatives ship them back?