Napoleon said:THIS!!
Don't exchange at baggage claim.
Change money when you are just about to exiting the secured area (Banco de la Nacion on your right) or just after you exit it (by turning right and then right again and going to the backside of Banco de la Nacion).
Good advice.
You won't need coins, because you're NEVER going to take the bus.
Why are you so sure? He might want to take a walk on the wild side of urban transportation. I mean, a ride.