Why not delete the XP partition so she has more room for Linux
Ok I assume that originally you reduced the XP partition size, installed Ubuntu and setup GRUB as the boot loader?
If so, you can delete the Ubuntu partition and resize the XP partition back to full size. At this point your machine will be unbootable as GRUB will try and start but fall over as it can't find the next stage of the boot loader (it was in the Ubuntu partition).
To make the XP partition bootable, you need to boot from your XP install disk, go to the recovery console and then repair the Master Boot Record (the MBR). You might want to Google around for more info on that as it's a Windows thing.
Usual warnings; back up everything and double check the instructions before you start.