The iPod can't playback WMA, which is the predominant format on Zune Marketplace. They do offer some DRM-Free MP3 songs, which her iPod will play. However, I don't know how much or what kinds of music are offered in MP3. Wikipedia says 90% of Zunes tracks are currently being offered in MP3. As long as you stick to DRM-free MP3, she can use songs from Zune Marketplace on her iPod. Just copy them into iTunes.