As spudqueen says art can be a very personal thing. Why not do some asking around her family to try & find if there is any thing she especially likes. You can always ask them to keep quiet if you want it to be a suprise & its better then getting a print she might hate.
There's no guarantee that they're going to like your choice of art.
But buy a print of a UK artist and find out a bit about them so you can inform your friend which in turn will make the art all the more appealing. And make sure the print is a good size!