Maizey:
You've answered your own question, "How did he take the law into his own hands?" in your original post where you said: "my friend jumped out of the car with a hand batton(sic)(he kept for protection)".......
1. The fact that he had that weapon means in law that he's in possession of an offensive weapon.
2. By jumping out of his car with it to confront the other guys is taking the law into his own hands.
3. The very fact that the others were armed with a bat and a knife is no excuse for your mate jumping out of the car and doing what he did. Why didn't he just drive off?
4. There's clearly more to this than this version. I'm speaking from experience of such matters.
The laws of the land are not designed for do-it-yourself merchants. What would have happened if your mate had come off second best, or, God forbid, been killed? You'd then have been mourning instead of moaning !