Your friend, well he does embody everything wrong with some gamers these days but not for the rage quitting... it is for the trollish behavior of having to find people weaker then himself to push around.
Personally: I prefer to play against people that can kick my arse. Given the years on years of FPS gaming I have under my belt, I consider myself to be fairly good when it comes to this genre. To come across someone legitimately better then myself is a nice challenge and I am more then likely to befriend these sort of people just so I can keep track of them. I want to know where they are, so I can play against them more in the future. Why would I want to play against someone better then me? So I get better at the game myself, of course! You never learn and progress your skill level playing against people you can beat in your sleep.
This sort of mentality also leads to having the teams best interest above your own. I see no problem being the 'polish mine detector' if it means the person behind me doesn't have to worry about getting his own arse shot by a sniper that he now knows the location of. I am more then willing to sit back at the objective even if it means I only see battle once or twice in the whole game, just to ensure no one takes it from us. It would be my pleasure to guard the entrance to the sniper nest, if it means you can focus more on taking out those attacking head on without worrying about your own back.
This is because I care more about my side winning the game, and often the mini-accomplishments towards that goal, then my personal score. Sure, I can say that because my K

often is in the positives even with this mentality, but you will find the game more fun if you keep the objectives above personal gain and strangely, those personal gains follow more and more after you realize an organized team beats a disorganized one every time. This is because it leads you into a whole new area of gaming: tactical planning!
You see, those mini-accomplishments mean a hell of a lot more then any K

rate. After all, you will always forget what your K

rate is by the end of the next round, but when you take out two enforced defense positions and achieve the objective through superior tactics? Well it sticks in mind for a lot longer and is far bigger a boost to your moral then just a good K

rate which you can get by hiding in a lone corner and picking off a few stragglers.
I still have flash backs to the time where we choked an enemies advance so heavily they couldn't move two feet towards the goal and not because I killed 15 of them in a minute but because the whole team worked like a greased machine to make it possible. I still shiver at the times where I was part of a two or three man team working so well together that the enemy couldn't organize enough to defend a single objective, let alone the whole map, from our hit and run strikes. Often with me being nothing more then the guy who is defending the choke holds needed to ensure my team mates have a safe path to constantly push against the enemy regardless of our own losses. I can not even count the times I get set up in the enemies base as a engineer on TF2, getting only one or two kills because of how slow the action is down there but single handedly making it possible for someone else to cap the intelligence and be back to pick up the next lot within ten seconds. Doesn't matter I usually end up half way down the list in points then... I know I was far more valuable to the team then just K

rates.
And isn't that what matters, those periods where you do so well that you cream your pants just thinking back on them?
As for rage quitting itself... well this is a game. If you are not having fun playing this game, then leaving it to find something better is not a bad idea. I, personally, do not hold a rage quit against someone simply because they are not having fun. I have even done it myself at times because I believe the number one thing of any game is entertainment. If I become to demoralized by outside elements to the point I can not focus on the game, if I end up feeling useless to the team and feel someone else should take my slot that might help better or if I'm just running into a wall of cheaters and no skill can be gained from over coming them... well why should I stick around just because it is considered somehow faux pas to leave?
Your friend... yeah, he is a troll and nothing more.