Hi Kasakato,
How many guppies are in the aquarium? Aggression among the fish can be related to overstocking the tank. There is not enough room for each fish to have a "territory", so they can fight. The fish may look fine for now, but if your friend can get another tank for the guppies, he should do so. If you have an overstocked tank, you most likely will have problems with parameters and aggression.
With livebearers like guppies, it's a good idea to have more females than males. Usually 3 females to one male is a good ratio. That way, the male doesn't repeatedly pester the same female and stress her out. This could also be what is happening in your friend's tank.
Can you take the harrassed guppy out of the tank? If you don't have another tank, you can put the guppy in a Rubbermaid container with an airstone. Then you can monitor it and see if it needs some meds, or just a rest in
QT for awhile.
The best thing to do in this situation is to get another tank for the guppies, so they are not overstocked in the first tank. Otherwise, this aggression could get even worse, with more fish harrassed.
