"Came upon"!?! Does that mean he bought them from someone? Or that he captured them in the wild?
If the latter he may well be guilty of a very serious crime. He would do best to simply release them.
If the former then he is probably going to need some documentation from whoever he bought them from, as well as from the PNG government
It may be an indication of a sad world, but it is a fact that in today's world, transporting exotic pets
back and forth between various countries is going to be an extremely trying exercise involving a great deal of paperwork and a lot of fees
(not all of which will be the "above the table" type!).
As an example, I have a friend who brought a Macaw back from Brazil
about 15 years ago. She started out trying to do it the legal
way and finally gave up in frustration as the day of her departure got closer and closer, and every form she filled out seemed to spawn six more that had to be dealt with. She ended up paying bribes totaling about $800 to several different officials in Brazil
in order to get her bird out along with the proper form to let her bring it into the USA.
Unless your brother is extremely attached to these parrots (as my friend is to hers), my advice would be to leave them behind.