New 50cm tank is going to be a waste of money. Pacu grow to a meter in length and 60 lbs. Even the 2.5m tank is going to be nowhere near large enough to keep all of the fish he has. Arapaima grow to 2m in length.
I understand that you don't want to rain on your friend's parade, but he needs to understand what he is doing is animal cruelty. Tell him to sell his fish. He is not going to be able to get a tank large enough to keep all the fish he already has unless he builds a public aquarium style monster tank. There is a reason Arapaima are not sold commonly in fish stores and have to be ordered- they are a specialty fish which should only be kept by exceptional fishkeepers with gigantic tanks. That is not your friend.