When two guys are walking together and one guy is listening to his headphones near them, those two guys are friends and the other guy is an enemy. When two guys are alone and listening to their headphones and walking down the street near each other, they aren’t enemies or friends. When two guys are alone and listening to their headphones and they attempt to use their metrocards to gain access to the subway and they are both rejected by the same turnstile, they are friends.