Need Help with Guppies

My neighbor has a tank with Guppies but he keeps telling they keep killing each other.
Are these guys aggressive towards each other?
He likes to keep no more than 5 in a 5g tank.
Thanks

No they are not aggressive to each other. I have 4 in a 10g and my 2 males chase each other but do not harm each other and the 2 females are so easy going.

I have a 10g with 9 males and they do just fine. Never had any of them kill the others.

Have a couple a bachelor pads and no issues with aggression

Does she have any place for the fish to hide? They are not aggressive, unless she is not feeding them properly. Sometimes you have problems with them eating young. This doesn't happen if they are fed well and the babies have hiding places. I had 6 in a 20gal. 6 months later is was more like 106. Certainly not killing each other.

I second everyones answers guppies are none aggressive, maybe her guppies are dieing from a diesease. I seem to think they tend to carry alot of them, just depending on where they were purchase as could be said with any fish.