She will probably have the windows installation code on a sticker on the laptop. If it does you can use any disk as long as it is the same version, i.e. home or professional.
sometimes there is a restore disk with all the other disks she will have received when she got the lappy. this usually creates the partition and installs the software from there. just make sure everything is backed up, as the restore software will destroy everything on the drive, and getting it back could be a pain.
M.