Hitting her with Unity is not a good start as that is totally alien to an XP user.
As suggested much earlier, lxde or xfce desktop would be much better (and faster than what you have now).
But I totally agree with much of what's been said. Dual-boot and let it be; if it's her computer, end of story, etc etc. Because you love the OS doesn't mean she is ever going to.
Best option? Try to get your hands on an old computer, install Ubuntu (or a faster flavour like Xubuntu or Lubuntu) and wipe Ubuntu from her machine.
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