Not sure if I totally understand, but I think what you're asking isn't really possible. If you want to modify the behaviour of a game, then that game needs to support modification. Some games expose scripting interfaces, some don't. Using Unity's scripting interface will not help you modify the behaviour of a non-Unity game. (It might help you prototype the solution, but it won't get you any closer to modding the game itself.)

Even if that were possible, which it isn't, for the reasons yoyo pointed out, you seem to be asking for a way to implement an automatic Headshot-bot. I wouldn't help you cheat at multiplaying if my life depended on it. Okay, maybe I would if my life depended on it, but you get the idea.

Tell your friend he's a lamer, and he needs to learn to aim instead. :)

Well I suppose it's all about understanding the logic of the game itself. It's not clear if your friend (really?) ;P is developing another game or is trying to, as Christian points out, cheat. If it's the later then no, it's improbable nearly impossible and should not be attempted as this would give your friend eternal bad luck and/or infinite ass pain. These are facts.

Yeah, thanks a lot, I ask this question because in my opinion I don't like to cheat(I think my friend yes :P) but I think this may be very interesting to learn because all the time I just get into Unity, Unity and Unity but I always had the question that if this kind of script can be used out of Unity. But thanks anyway for the suggestions and replays.