I'm not really a part of the Pokémon fandom, but I am a big fan of the main series of games. I don't follow the show anymore (haven't since the second season a long time ago,) and I don't interact with the Pokémon fandom.
I like Pokémon, because it's a simple RPG that allows you to travel and overcome challenges. It also has a ton of customization in terms your team of fighters (i.e. your Pokémon.) It always had nice, upbeat tunes. And it's a universe that you can immerse yourself in. Outside of the fact that there are obviously evil syndicates (that you eventually take down yourself) and the fact that they're are nearly all-powerful monsters, many of which are running about wild, the world is peaceful and fun.
Kinda like Zelda, you have the opportunity to explore and adventure a world that's a lot more interesting than yours usually (unless you find this world more interesting, but that's often not the case in Web culture.) I think that is the game's appeal.
But I think your friend may have been a person who simply gets wrapped up in some things. So when he found a game that he really liked, he chose it over his friends. It's sad, but it happens. I'm not one to say that it's obviously a bad thing when you choose a game over people, because that's a choice that I don't often make and most certainly on a regular basis, but some people would. I hope that doesn't feel like an insult to you. Your friend just had odd priorities, it seems.
I still don't get Touhou. I get the game, but I don't get the appeal. Perhaps I would have to read the stories behind the games, see some of the fan content, or talk to people of the community (the only user I know would be Natsuru, and Natsuru is as cool of a customer as they come.)
But I don't see how a fandom sprung up from it. The article didn't really help me understand it.