Spoilers for best friend romance ahead. If you've finished the milkshake mystery plot then all that's left to do is to visit their dorm and talk about sex. A scene should trigger where they're watching mature videos (not sure what are the terms on language use on itch), you can ask to join them, and you end up watching videos together until you fall asleep. After you wake up a scene where you can talk about your relationship will happen. If it's not happening then it's either bad rng or a bug.