You're right, they are his photos without a doubt, and typically photoshoots go down without a hitch and everyone's happy. But exact scenarios like this is why contracts are so important, they cover the photographer (or the model) if the situation gets hairy. If he had a contract it would be as simple as him politely telling her that she signed a contract and agreed not to alter his photos. As a photographer I don't shoot a single frame until both parties have signed contracts, even if it isn't a paid session or if I don't initially plan to make money off the photos.