
If that person used the product key to install Office in their PC, then the Office licence/subscription is now associated to their Microsoft account, there is no method provided to move a Microsoft licence form one Microsoft account to another.
The only option because this forum is just a user to user forum and no-one here has any account access will be to contact Microsoft Support directly, provide them with proof of purchase and the product key, and they will be able to move that licence for you.