How big is the tank, what is your water changing schedule like, and does he have a heater? The main cause of fin rot is poor water quality--this means that you are not changing the water enough. Fish constantly excrete ammonia through their gills--kind of like urine. This quickly builds up in small tanks and can burn the fish, causing delicate tissues like their fins to deteriorate.
The best way to deal with this is through clean water and stable temperatures--if you have not already, you should do a 100% water change with dechlorinator. If you don't have a heater, you should definitely purchase a heater with an adjustable temperature dial.