Does my American friend need a return ticket to enter Brazil?

I used my air miles to get a one way had no problem. But this was over 4 years ago. With all the closed boarder around the world to Brazil from travel. Need to call your airline company. They will be the one who will stop you from boarding.

US citizens are on a visa waiver with Brazil, and the Consulate sites (I checked Chicago and Boston, which tend to keep their data current) don't mention a round trip ticket requirement for visitors in that category.  Rules change frequently with the pandemic, though, so your friend should keep checking the site of the Consulate responsible for his region.

Airfares are crazy, so he should also price round-trip tickets, whatever his plans.  When I moved down, I didn't need a round-trip ticket, but I bought one anyway, because it was $400 cheaper than a one-way!

Many countries (Brazil is on the top of the list) can not re-enter without covid neg test and can be quarantined after entry,
Normally, no return ticket is not required for US citizens. WIll be asking for a hotel reservation, enough money to cover expenses (without return ticket money for 90 days).
Recently friends (one a citizen the other his Brazilian spouse) waited until after Jan 26. Citizens could enter but since connecting via Mexico city spouse had a 15-day quarantine.
The big issue is looking into the future again. Flights can be cancelled from BR as they currently are now and in the past, some flights had to enter certain airports and then some had quarantine hotels before being able to go to their home city.
Consulates are the last to hear about entry requirements. The Presidents (Trump and Biden) are using "executive orders" to control covid entry. They can be found on Homeland Security/CBP and State Department.
You did not ask, my 2 cents, do not travel, Think 8-14 hours in an airplane, various airports, transportation. Health insurance (covering covid and even though some cities had no beds)

Thank you very much! Really usefull information