Overpowered? Absolutely. Loads of fun? You bet. Stacking the deck? Probably. A cheat? Certainly not.
Thing is, unrestricted leaders is not a default option. Firaxis balances the game for standard play, not for every conceivable possibility that somebody could do with it. Boudicca of the Romans is therefore no more a cheat/exploit than, say, Pericles of the Ethiopians (huge culture boost) or [insert your favorite combination here]. It tends to one particular style of play which doesn't allow for a change in plans. Suppose you don't get iron, as somebody said, or suppose you pursue a cultural path, then get attacked from all sides or fall too far behind in techs to make wonders.
No, it is not an "exploit" in the traditional sense. Unfair, maybe, but if you don't like it, don't play with that option. Plain and simple. If it were the standard game, I might say yes, but it isn't. The real issue here is that the AI is not smart enough to contend with it; play on multiplayer with Boudicca of the Romans and see what happens. More than likely, you'll face smart opponents who will keep you in check.
You say "stacking the deck" equals cheating in your book. I won't disagree; however, that is effectively what you are doing when you play with unrestricted leaders. Who in their right mind is going to pick a horrible combination, unless they want a challenge? That's why people use this option: because they want something outside the traditional boundaries of the game, which will liven it up a little. It stands to reason that certain combinations will be more overpowered than others. Is that bad? No. It is, rather, a feature.
To me a cheat is something that is against the rules of the game which will enable you to win when you otherwise shouldn't have, based on skill and whatnot. Playing as Boudicca comes awfully close to this, so while I don't consider it an official cheat, I would consider a win with the Agg/Cha/Praetorian combo to be less satisfying than one without it. Hence, I'd tell your friend to have fun with that (because that's the point, isn't it?) but when he gets ready for a real challenge to come back to the normal game.