I'm not sure what she was expecting, since she knew he wouldn't be revealed as a genius for 3 more years. It would be weird for his style right now to be the same as his style later. And with something as fickle as art and inspiration, she might have either sped up his revolution, or completely derailed it. Trying to force someone to get a breakthrough ahead of schedule might just send them in the opposite direction. It could be that he needs to go through some personal event, have some kind of revelation before he becomes the person he would be three years later.
As for not paying, she asked upfront how much it would cost and he said she could set the price afterwards. He didn't seem to have any other customers (it is already fairly late in the day and she was his first), so there was no opportunity cost, she didn't like the picture for valid reasons (lack of quality which even the artist didn't dispute), and she didn't take it with her. So why should she pay? Most street artitsts generally have samples of their work for exactly that reason, so the prospective customer has some idea of what they're getting. If he'd had samples of his exaggerated style for display, she could have saved herself two hours.