He probably needs to first run the slmgr commands to make his install forget that it ever had a KMS activation. Once it is completely in a non activated state it should take the retail 8.1 Pro key and run with it.
No, but he said it worked. And you and I know that KMS to gen will work, but ABB said it was the previous retail key.
I had KMS Pico generated key, that's why it wouldn't take my legit 8.1 Pro key.. after changing the key to generic 10 Pro one - i was able to use my 8.1 Pro key and activate, it's simple.