They aren't housed together, right? Because unless they were all females that would be the problem. The other problem is he most likely didn't cycle the tank before putting fish in. Unless you use Bio Spira this takes months.
In a new tank he should mostly be testing for ammonia and nitrites...both should be zero or the fish can die. If he doesn't have a water test kit he can get it tested for free at a petstore.
What size tank did he have and what kind of filter? Also, what was the temperature?
Did he dechlorinate the water? For the fish to die that fast there was definately something poisonous in the water...probably ammonia, nitrites, chlorine, or chloromines.
Here's an article about cycling an aquarium: http://www.fishforever.co.uk/cycling.html
Also, here is a link to buy Bio-Spira: http://fishstoretn.com/bio_spira.html
You shake the Bio-Spira and then add it directly to the filter cartridge or bio-wheel at the same time as adding the fish. It is the only product known to cycle a tank overnight when used correctly in a new tank. Don't let the salespeople talk him into buying Bio-Zyme and similar products because they take weeks to cycle and aren't worth it in my opinion except maybe as a supplement.
First he should change all the water and wash everything before adding new fish since there's obviously something bad in the water.
Does the tank have a top? A top may help to keep his children from adding stuff to the water.
I hope this helps!