Kicking the guy in the head just as the police arrive on the scene, so all they see is his kick and not the punches that preceded it, makes it less than a "clear-cut" case IMHO. It may have been perfectly righteous (certainly it sounds like it in the telling by your friend), but it is hard to judge a case without having reviewed all the evidence. Maybe there were other facts that made the plea bargain advisable, facts that your friend may not have mentioned.
In any event, while the advice to find a lawyer before you need one seems sound, in reality, few people are going to invest the effort. Moreover, most lawyers won't be inclined to provide a free consultation for someone who does not need their services and may never need them but who wants to talk, just in case.
I think it would better serve gun owners to know the law of self defense in their state. For example, in some states, a party has the duty to retreat if he can do so safely before he uses deadly force to repel an attacker. In other states, no retreat required. In some states, retreat while inside your home is not required, but outside the home, it is.
Where to find such information? Sad to say, you're going to have to work for it if you don't want to pay a lawyer for the information (and I don't think it would be worth the fees to do so). The best bet would be to go to a public law library, such as the county law library if your county has one. Look at the treatises on criminal law, and review the chapter on self-defense. If possible, look for a treatise that concentrates on the law of your state. If all the criminal law treatises are general (i.e., have a national rather than a state orientation), learn what you can from them about the law of self defense generally and take the most conservative approach.
While you may be able to find worthwhile articles on self-defense on the internet (did someone mention Google?), be careful about getting advice in online forums. IMO, the legal advice given on internet forums is worth double what you pay for it.
--Rick