Registration Key Issues

My friend used my computer to play the demo version of altitude, so when I bought the full version it automatically registered the full version to his account when I started the game up. After realizing that I had to switch to my account and register the game, it was already too late. Every time I try to register the game, it says that it is already registered under his e-mail. I'm not sure what to do, and I don't want to re-purchase the game.

Also, his account was already registered for the full version of the game on his computer, which makes this even more confusing to me because I didn't think the same account could be registered twice.