By casting that spell, didn't he automatically lose the duel? It wouldn't matter if his familiar won. Though I doubt that it will matter that the spell is in violation of the rules it'll just be handled. It would be the crushing defeat he deserves.
Technically yes. At the same time,
technically losing the duel because you got disqualified for killing your opponent and actually losing the duel because your opponent was just better than you gives out two very different reputations.
Both are blows to your dignity, honor and all that, that is, you lose face, but one loses a lot more face than the other, specially if you care more about the reputation of strength than that of honor. And that's before the possibility of some aristocratic corruption trying to cover it up and retroactively declare that "it was actually fine" and either say that the arbiter didn't say what he said or was wrong in his ruling.
Now, of course, it will fail miserably at actually harming the protagonist, so not only he'll lose face for using the forbidden technique but also lose even more for doing so and then
not managing to defeat him anyway.
As a sidenote, if the protagonist really wanted to clown on the dude, he could also go for the "let
me lose from a technicality" by, say, completely dominating the fight with zero stops, and then just before the final blow, disqualifying himself by asking Iris if she wants him dead or not. Not much point in doing this in the particular context this fight is taking place, but it'd be funny, and he'd certainly be even more pissed aftwards.
