There are many factors a police officer takes into consideration during an arrest. Without knowing all the details, it may be that your brother's friend was seen as acting in self defense. Self defense makes a difference in assault cases-

Arizona laws define assault as: causing any physical injury to another person [either intentionally, knowingly, or recklessly]; intentionally placing another person in understandable fear of receiving a physical injury; or knowingly touching another person with the intent to injure, insult or provoke such person. If someone commits any of these, they have committed assault [A.R.S. 13-1203]

If someone committed the act of assault, he/she may not be found guilty if he/she has a legal excuse for committing the act. This is where self-defense comes into play. Arizona laws say that 'a person is justified in threatening or using physical force against another when and to the extent a reasonable person would believe that physical force is immediately necessary to protect himself against the other's use or attempted use of unlawful physical force.' Basically, that means if another person is about to assault you, you may defend yourself from that attack. Your defensive act, although an assault itself, may be excused in these situations.

Keep in mind that the excuse of self defense does not apply in response to verbal [spoken] words alone, while resisting arrest from the police [regardless of the legality of the arrest], or if you did something to make that person assault you [unless you take back whatever it is you said/did to make that other person assault you AND that other person still tries to use unlawful physical force against you].