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#11
A volume license is a group of licenses purchased by a corporation at a bulk discount so they can license all of their computers running on their network. KMS (Key Management Service) is one method of activating volume licenses. The corporation will have their own activation server which is licensed to activate a certain number of computers. The client computers will have a certain product key installed, and for Windows 8.1 Pro it is GCRJD-8NW9H-F2CDX-CCM8D-9D6T9. The product key for KMS activation requires the computer to contact the corporation's KMS server periodically in order to remain activated.
Now, I seriously doubt that your friend's computer is a member of a corporation's internal network and legitimately getting activated by that corporation because then it would be your friend's corporation that the computer belongs to that would be responsible for upgrading it to Windows 10. So that leaves the probability that your friend is using an illegitimate KMS activation scheme to keep their Windows 8.1 Pro activated because they did not pay for a real license for Windows 8.1 Pro.