To be fair museums themselves make this worse. I still haven't come across a museum (granted I don't go a lot) that had exhaustive good-quality photographs of all their exhibits available online.

So to give this girl the benefit of the doubt. Maybe this is the only time she'll be in London, she only has a few days, and she's genuinely interested in the art, but taking photos of it to inspect it later allows her to enjoy more of it than hanging around carefully looking at individual pieces at the expense of not seeing any of the others at all, ever.