Why would keeping a silver arowana in a small tank make it die? The fish could grow to around 18" in that tank no problem, and will not suffer from any negative effects. I do think that this tank is too small for a silver arowana, but you all make it sound like the instant he drops the arowana in the tank it will die. This is not true, many people keep arowanas in tanks that are too small for longer than they should have. I've seen large silver arowanas crowded into tanks at pet stores living in there for months, even years. I can see why he bought the silver arowana, they are magnificent fish and one day I hope to have the fortune of owning one. The one problem with these fish is that they can reach 4ft long MAXIMUM. Not silver arowanas get to this size, most never even get over 3 feet, especially in home aquaria. They do need large tanks, but living in a small one for a year will not kill it.
Personally, I think a silver arowana should be in a tank atleast 8ft by 4ft wide. This is for an adult size of 3ft, If they do not get that large, then smaller. Keep in mind this fish at 4ft in length does not need the same space as a 4ft cichlid, silver arowanas have bodies similar to snakes or eels. They can turn very sharply and comfortably despite the length, compared to any other large fish of the same size. Do not quote me and say I said that arowanas are related to eels or snakes, I just said they have a similar body.
As for the owner, I can understand your doubt in him purchasing a larger custom tank, but please just assume he is. This means the owner knows what he purchased, and wants to get an aquarium where he can house the fish properly. I'm sure if the owner sees that he cannot get this custom tank, he will find a new home for it. Please be less hostile.
Oh, and mark, try to use some punctuation.