a key is stored in the UEFI but not displayed anywhere directly. that key is specific to that hardware layout and the version of windows it came with and will automatically register itself any time you do a bog standard reinstall. you have to fuck around with the original downloaded image to force a different version of windows than what came with the machine which is not difficult but then wont automatically register, for that you need to buy a key or use a key management service.

most people dont know how to extract the key from the UEFI to begin with.

a more correct response would be giving him the instructions to do it through powershell - eg 'wmic path softwarelicensingservice get OA3xOriginalProductKey'

just tested that on my machine and sure enough, it returns the original Windows 10 basic key and not the Windows 10 Pro key generated by the kms.

Both compromised.