A guess would be Aeromonas or a wound that was left untreated and became infected with Saprolegnia. It's tough to sat for sure but those two are good bets. You should star by having your friend test his water, ammonia, nitrites, nitrates and PH. This is generally caused or made worse by poor water quality. Do a 25% water each day for the next week. Vacume the substrate with each water change, clean the filters in some tank water and add 1 tablespoon of salt per 5 galons of water. Remember to raplace sal removed by water changes to keep the level constant.
If the other fish start showing any signs of infection, post water parameters and we can then suggest a med.