The way I see it - from the sounds of it - the older brother had an unfair physical advantage over your friend, logically how is the guy going to protect himself? From the sounds of it he wasn't in the position or of capability of defending himself with either knowledge of fighting or physical strength...thus the attacker had an unfair advantage that meant the defender couldn't actually do the defending without intervention or the use of a weapon. I know the justice system doesn't want to encourage weapon use, but I'm sure it doesn't want to discourage making last resorts on defense?
From what I understand from reading this, he did use the weapon in attack, but as a threat, if he had intended on using it, he would have, surely.
Though if he could escape or wasn't at a last resort, it would more understandable why he received what he did. Though it's really hard to judge, we're getting a voice from you from your friend, so dare I say it, a level of it is going to have some kind of bias in the story and it sounds like the story is one person's word against the other, which of course makes judging the actual incident and motives difficult, even for the police.
Though 6 months is a little much, and I hope he doesn't get that. But hey, it depends on the quality of your justice system there, individual factors of coppers involved and how they see it.
"Experience never provides its judgments with true or strict universality; but only (through induction) with assumed and comparative universality." - Immanuel Kant
