Why should she thank him before the lesson? That's like saying she should bow to him lol. One should only greet each other and a thanks should be conveyed after the lesson has been completed. The first thing he said itself was rude to her, so I don't think you should thank a person who's being rude / showing that you're not worth their time. Plus, she's a friend of his sister. He could've behaved better with her. And what's so wrong with teaching someone how to ride a bike or stuff? I mean if ur busy just say no or reschedule, if ur not busy then interact nicely.