It will work with any Amazon account. Using the Fire TV Stick, go to Settings, My account, click the centre button on the Remote and you get the Deregister option. Click again and then your friend can register it to his/her own Amazon account.
Or, on your Amazon account, go to Content and Devices, select Devices and then select your Fire TV Stick and deregister it there. Your friend will then have to register it to his/her Amazon account.