The Windows 8.1 installer from the ISO does not accept Windows 8 keys. You need to install the system with a generic key (334NH-RXG76-64THK-C7CKG-D3VPT for Core, XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB for Professional) and then enter your Windows 8 key after installing.
he won't need to install win 8.1 again - he can just enter the new key
however, looks like this trick might do ... unless OP has already done it and it does not work