Why are you planning on paying for music? Get her one of the million free user-to-user things, explain to her that she should click the music radio button unless she wants tons of porn, and take her out to dinner with the money you saved.

If you're really concerned about legality, I've heard Napster's new paying system sucks and that they make it really hard for you to transfer the music. I have a few friends who pay for music on iTunes and love it. This is second-hand information, though, and I really think you should just get the music for free.