You can buy Microsoft Office 2013 (Home & Student Edition) as a download:
Amazon.co.uk User Recommendation
However that simply looks like a good way to waste a hundred quid to me because she doesn't need Microsoft Office anyway! OpenOffice is just as good, and totally free:
https://www.openoffice.org/
OpenOffice can handle all Microsoft Office formats. However it's worth noting that the default settings are for it to save files in its own native formats. You can use 'Save As' (instead of 'Save') to get around that but it's far better to simply change the default settings so that OpenOffice automatically uses Microsoft formats to save files anyway. Instructions here:
http://helpdeskgeek.com/office-tips/set-openof%EF%AC%81ce-org-to-save-in-microsoft-of%EF%AC%81ce-format-by-default/
OpenOffice isn't the only free alternative to Microsoft Office. LibreOffice is also highly praised (and has been developed from the commercial Lotus 123 office suite):
http://www.libreoffice.org/
Kingsoft Office is 'the new kid on the block' among free office suites but it has quickly won many fans:
http://www.kingsoftstore.com/