Chocolate actually is finicky material. I've ruined some of it, myself, in the past, while trying out things using just my imagination, not instructions. However, Nanami here has Sora teaching her, so she should have been okay. Nevertheless, it's jolly good Sora pushed her to give the chocolate. It's quite natural they would view Riku a bit differently since Sora is being doted on by him every day, whereas Nanami is just being treated well as precious childhood friend (and maybe a bit more).