Now really. That guy accepted that random girls help, day for day. What did he expect, why she was helping him? At least you have to take in consideration, that she might have romantic interest.
Edit 04/14/19: And no, he doesn't owe her a relationship, but it's still kinda not nice to not tell her, that that song is for another girl. Come on guy! In most cases, when somebody unknown from the other sex offers his/her help, it mostly (not always, but very often) boils down to them wanting to gain favour with a very certain goal in mind. Not taking that simplest of natures motivations in consideration is either beyond stupid or a dick move. Then again he confessed to the girl he likes with a song and in the next moment he waves to that other girl that helped him with practising it with him...I see he really has a knack for this.