1. Fish being stressed or in poor condition for some reason, e.g. rough handling, fighting, overcrowding, poor water quality, incorrect water quality, very low temperatures (below 50F).
2. Secondary infection following physical injury such as nipping by other fish or wounds from tank decor, or following a parasitical infection.
Treatment:IRemove affected fish to hospital tank if possible. Use a broad-spectrum antibiotic or antibacterial medication (e.g. Maracyn and Maracyn 2 or Kanacyn in US, Myxazin in UK). A .3% salt solution may cure mild cases. Perfect water quality is essential for fish to recover. Healing fin tissue can be aided by using Melafix.
Precautions: Finrot can usually be prevented by correct care of fish, e.g. maintaining good water quality and correct stocking levels, not keeping 'nippy' fish with goldfish. Antibiotic medications affect the cycle so monitoring water quality during treatment is essential.
I hope this helps your friend's fish.