Frankly, he needs to increase the size of the tank to 5 Gallons and put a betta in it. There are no fish that will be happy in 1 Gallon.
Generally to treat water use a good conditioner, to take out chlorine, chloramine and heavy metals - Seachem Prime is great. You also have to cycle the tank to the point where the available bacteria can process the fish waste from ammonia, through nitrite to nitrate and then manage nitrate by water changes. In the event that the water contains ammonia from the tap Seachem Prime will take care of this.
But please tell him, no fish in a 1 Gallon 'tank'...there is no swimming room, no room for a heater, no room for a filter, parameters will be impossible to maintain...his son will just end up with dead fish.