There is no gaurantee that you friend will avoid arrest by getting legal representation. However, your friend's chances are certainly better with an attorney than without. Self-defense is a very technical defense full of exceptions. For instance, even if your friend did not initiate the fight, he might lose a self-defense claim if his response was excessive (used excessive force) or continued after the threat was gone (beat someone when they were already down), etc. Therefore, your friend needs an attorney familiar with the legal requirements of a self-defense claim.