To tell you the truth... there's not a whole lot of information floating around the web that you can't sync the iPhone with more than one computer. Especially if she had an iPod or iPod touch before buying the iPhone. I upgraded to iPhone from iPod touch and I assumed that it worked the same way and I didn't realize it was different until after I bought the iPhone.
You would think it's common sense to think that if the iPod touch can do it, the iPhone can do it too. Afterall, the iPhone is like a super iPod touch with a camera, microphone and cellular data.
I have no idea why they restricted this but it gets on my nerves.