Legally, it's not a very good idea to carry a knife, as Trumble has made clear earlier in the thread.
The scario that happened to a friend of somebody I know was:
A and Co. jump on B's sister
B punches A and Co.
A is severely hurt
B is currently the one in trouble with the law, despite responding to the situation in an entirely reasonable and justifiable manner.
From the facts you've shared, perhaps A is trying to (mis)use the 'Thin Skull Rule' to his or her advantage.
The Thin Skull Rule is, if A strikes B's head without intending to do him serious harm, but B nontheless suffers serious harm because of his abnormally thin skull, A is still culpable for any harm caused, even if A was unaware of B's abnormality at the time.
In real life, B does not necessarily need a literally thin skull - just something that makes him abnormally vulnerable. I don't know if this is the case with your friend. In the absence of detailed information, I'm simply speculating. So don't take this particular section of my post too seriously. I'm just thinking aloud. Or onscreen. You get the point.
Regardless, in your case, B's legal representation could reverse the situation if they do their jobs properly and persuasively argue that B was acting in self-defence and presumably did not intend to seriously harm A.
better to carry a sword or a gun brother...
If he wants to get arrested, yes.