Knowledge and then the application of knowledge is really the best way to succeed in anything. I feel if your friend did a lot of studying and really committed himself to getting better that he would. This doesn't mean playing 20+ games a day and grinding it out, this means reading, watching, taking notes, studying those notes, applying the acquired knowledge over many, many games, watching and assessing replays, following any hint of new and useable information about the game (pro streams, blogs, forum posts, adding players who seem to really know what theyre doing and asking them questions). Look for free coachers like Worrdragon, put all of his interest in just this game. Pro players (and probably most platinum+ players) have a good chunk of their day-to-day life invested into this game. It's more than a hobby to those who aspire to and reach the highest levels.
Also, once you become a true professional or truly talented at anything through the merits of hard work, motivation, and inspiration, you've learned HOW to learn, so it becomes applicable to anything in life. This is probably the case with lots of good LoL players, they're simply better at learning from their mistakes and their victories.