Strange CD key mishap

[MorOs (DayZ) 0]

So me and my bud generally play DayZ from our respective computers. However, he had purchased a laptop capable of running DayZ, and while I was over at his house, we decided to test out DayZ on it with me on his laptop and he on his desktop. He had Arma 2: CO installed and DayzCommander installed w/ the mod already DL'd, but had never run the game. We played for a bit, and he never touched DayZ on the laptop again. About a month later, we decided to play, but he was going to play on his laptop. So we found a server and he joined first, but when I was going to join, it would say I had disconnected and sent me back to the main menu. We moved to another server, he joined first once more and when I would join it said "CD key in use." After a few more tries, we figured that somehow he was using my CD key on his laptop.

 

Note that when I used his laptop, I logged into my Steam account and when he used it, he was on his own Steam account. I haven't had any problems with DayZ, save for that one instance, and have played just fine since. He'd like to be able to use his laptop when we play. Any idea how we can fix this?