A family of Microsoft word processing software products for creating web, email, and print documents.
Quickest workaround: Have your cousin open the document in Word and use Save As to save as a .doc (Word 97-2003) format document and resend it to you.
Starting with Word 2007 Microsoft introduced a new (and better) file format. .docx
They for some time issued the Compatibility Pack to allow Office 2003 programs to open these files but at some point stopped supplying that. You can, as Alvise suggests, download it from me. Suzanne advises that it has stopped working. You can purchase Office 2024 or Microsoft 365. You could also download and install OfficeLibre or OpenOffice, which are free, and which will read and edit the newer file formats.