The thing is here, there is a boundary between self defence and then the needless harm done to others, after you have made yourself safe from further attack.
It's like the story on the news last week, the man and his son chasing burglars down the street after one of the burglars held the family at knifepoint. They caught the burglar and injured him badly, permanent brain damage. This cannot really justify self defence, it is more revenge, hence the father was jailed for 30 months I beileve. If it was in the family's property then it would be a different issue, but they *chased* him, when there was no need to.....according to the law.
A similar situation arises here, in different circumstances granted. To be charged with three counts of GBH with intent, then there must be evidence that your friend went past self defence line and more into the revenge kind of things, which is when you start to get into trouble.
The fact that he did not instigate the fight will be taken into consideration, but if the court decides that he went beyond self defence then he will be convicted.
According to the law, GBH with intent is punishable with any term the judge seems fit, and could be anything. This won't happen though, it never does.
Since it was aggravated, there are no permanent injuries, the arm will heal I assume, I would have thought 4 to 15 months in convicted.