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Microsoft, unfortunately, is very strict and retentive about their licensing etc.
But, you could apply to them for a replacement CD for the license you still hold. "Them" meaning either Microsoft (unlikely to do more than offer to sell you a new copy), or Fenwicks (if they didn't provide the install CD with the issue of MS Office Student, they are strictly speaking not within their rights... if you purchase software, you have a right to the CD for it. Stores often keep the CDs, heaven knows why.
If your Mother-in-law is not absolutely hooked on Microsoft Word, she might be able to use Open Office instead - available and installable free of charge. It's Writer software is good enough for composing documents.https://www.openoffice.org/
There is also Google Docs, which is free for those who register for (or, already have) a Google account or a GMail account.https://docs.google.com/
Or you could compain to Fenwicks central customer support number, and ask for a return-and-refund arrangement for the PC in question. And buy elsewhere, better.