Well, the magic number is that "last 30 days" rating. Think of it like this...If I'm an 1840, then quit playing for awhile and timeout all my games, I could drop to 1145. Now, if I didn't play for 30 days then I could enter the rookie duel (as my "last 30 days" rating would be 1145). As soon as I start playing in the tourney, I smoke everyone and my rating skyrockets past 1250. This is most likely how your opponent got into the rookie duel.
Unfortunately, there are some unethical players on this site and you will see many threads on various tactics that they employ. What can you do about it- nothing - or maybe have Adam throw up the apple.
There always has been unethical people in this world, always will be.