Even if he flashes the ECU to remove the codes, A. the codes might come back on their own, and B. the car will definitely not pass the sniffer test.
Your cousin has at least 3 options:
1. buy aftermarket cats and have them welded on to where the stock ones are, they range anywhere from 100-500-ish and should theoretically perform the same.
2. take it to a shop that passes cars for a price.
3. find used OEM cats on car specific forum.
either way hes gonna have to shell out some money, i'd just get new aftermarket cats or try to find used ones on forums like these, people are always taking off their stock exhausts and may have some.