I don't really think this has anything to do with the Austrailia forum....

The person traveling has the ultimate responsibility that the requirements are met to enter the country he/she wishes to travel. Australia passport holders don't need a visa but from another country one probably needs a C-1 visa.

The traveler is at fault although the travel agent should have explained the visa process if they were any good at their job. This sort of information is probably one of the reasons your brother hired them.

The airline gets fined (I believe) if a travel gets to the USA without proper documents so the airline is definitely not to blame.....UNLESS your brother was holding an Australian passport. Then they involuntarily denied your brother boarding without cause.

The only issue I see here is if advice on USA visas was part of the service your brother paid the travel agent for. If he didn't get the service he paid for, he's probably entitled to some sort of refund but I don't know how one would actually collect on such a thing.

I think it is common sense to look up the entry requirements of a country you are traveling to/through. I'd probably advise your brother to take generous 50% the travel agent is willing to give him and cough it up to experience. I'm sure he'll never make the same mistake again.