Will give you only one hint, when you are about to give her energy drink think out side of the box instead of just giving it to her. There's also hint in the dialogue option 'How can I see her butt..' to encourage people to find another way.
Come on, at least there's a hint that it has something to do with giving her energy drink. I told my friend same hint and she was able to figure it out and I think you could too.
