Here's a crazy story for you.
I've recently (well, 4 years ago) been involved in a serious attack.
New Years Eve some friends and I were out at the bar drinking and having fun. We left the bar and I was hailing a cab. Someone randomly started talking shit to me. I backed down because I did not want a confrontation (and because I had made an awesome chocolate mousse which was waiting for everyone at home), but the guy pushed me anyways. I still backed down and said "hey man, I don't want trouble" (mmmm chocolate mousse....). But them my wife said "don't push him" and he took two steps to her and punched her in the face as hard as he could.
I snapped. I was no longer in control.
A lot of things happened after that. Things ended with me smashing his head on the ground repeatedly and him almost biting my thumb off. This guy was an animal, he was on drugs, bucking like crazy under me and I only got lucky because I weighed more than him and watch a lot of UFC and knew to spread out. Once I got on top of him there was no way for him to get me off.
Here's the point. This all happened in about 2-3 minutes, but FELT like 1 second. I almost always carry a knife for random "I need a knife" moments (I didn't that night because of dress pants and it felt too heavy). I didn't have my knife with me that night, but never thought about it either. It all happened way too fast. Was too busy defending myself, defending my wife, and attacking that guy. Not to mention as I said above, I think I just got lucky to be on top of him, and spread out so he couldn't get me off and I could hurt him. But what if I wasn't lucky? What if I wasn't lucky AND I had introduced my knife into the mix? What if I WAS lucky and introduced my knife into the mix?
I'm an average guy and it took two cops to pull me off this guy after I threw one off me (I didn't know he was a cop) because in that second, in that moment, I was an animal, I was super-human. I had no plan, I didn't know when to stop beating him. I didn't know anything. It was auto-pilot. And I think that's what happens when you get into really serious fights. I actually scared myself a little bit.
If you had a knife and got into a similar situation, AND if you had time to deploy it, do you really think you would have the restraint to stop mid fight and say "OK, he's injured, that's enough?" Or would you keep stabbing and slashing? (I probably would have killed that guy with my bare hands given a little more time, and not because I wanted to. Just didn't know how/when to stop). Do you want to accidentally murder someone (right or wrong?). Do you want to maim someone because they drank too much or did too many drugs one night and picked on the wrong guy?
I'm glad I didn't have my knife that night. Even if I did I doubt I would have used it. And I feel bad for the beating I gave that guy, never mind the damage I could have done with a knife. I like having a crazy story about a serious fight I got into one time, and not the story about how I got jail time or narrowly escaped jail time.
Hope that somehow helps your friend.