This achievement is missable and occurs towards the end of the game, so spoilers ahead if you haven't played through the story yet.
After the fight where Milo and Lola get split up, whichever character you are goes back to 1st and Izzard to re-unite with their friend. Upon arrival, Sam is at the bar, and points out that your friend is over at the beer pong table. No, not your human friend, your other friend, Wormhorn.
Go speak to her before going upstairs -- if you go upstairs first, Wormhorn will be gone and you'll miss your shot. She's feeling depressed about the fact that her existence will be over soon, so be nice and offer to play a game of beer pong with her. She'll tell you to go get your friend first, so go upstairs to find out what your counterpart has been up to, then come back downstairs and sit at the bar to reconcile with them.
After you've hugged or high-fived it out, instead of just leaving the bar, go over and have your beer pong game with Wormhorn. Again, try to say nice things... I don't know if that makes a difference at this point, but it can't hurt. The achievement popped for me as soon as the beer pong game was over.