Not well for him. Kendo will teach you how to handle a katana. It won’t teach you how to use a sabre, rapier, or whatever stray implement of death and dismemberment was convenient.
It would be like asking, “my character has spent the last 25 years mastering Blackjack. How would they fare in a Texas Hold ‘Em tournament?” Yeah, they both involve cards, but it’s the wrong skill set, with an entirely different set of considerations.
So, if he’s not using a katana, all of his training is worthless. He’s literally trained in the wrong weapon.
If he is using a katana, he’s still screwed, both because of the weapon itself and because of his training. The katana is not designed to deal with the kind of combat Italian school fencing delivers. It simply isn’t agile enough. Generally speaking, the katana isn’t a particularly adaptable weapon. Take it out of its native environment, and it suffers.
His training is a problem because, well, he’s trained in Kendo. Kendo is not, and never has been a practical combat form. It’s the modern descendant of Kenjitsu, which was the art of murdering people with a katana. The problem is, as with a lot of European sword schools, Kenjitsu basically died out with the sword on battlefields. (Specifically in the mid 19th century for Kenjitsu.) Modern Kendo and Iaido salvage some techniques and katas from Kenjitu, but the result is similar to modern reconstructionist European techniques; potentially lethal, but nothing that compares practitioners from when they were living combat forms.
There’s an added wrinkle here: because of the social structures at work in Japan, particularly the insular nature of the class system, it would be literally impossible for your character to obtain training in Kenjitsu.
You know the cliche: “the right tool for the right job”? In this case a katana, and Kendo in general, is the wrong tool.
There’s also a pretty strong whiff of orientalism coming off this post. Best to kill that now.
There’s nothing particularly special about the katana. I know, this flies in the face of all the anime you’ve ever watched, but
when we’re talking about the sword itself? Technologically? It’s a
fairly standard early iron age longsword.
There’s nothing particularly special about Kendo. It’s a sword form. You can kill people with it, but that’s not saying much. The biggest problem with Kenjitsu is that it was an insular style. It evolved to deal with other people who were using similar weapons, held similar combat doctrines, and had similar training.
European sword forms evolved in an environment with far more diversity, and the result were far more adaptive combat styles. They expected people to pull weapons and techniques they weren’t familiar with. For your Duke, the question wouldn’t be, “what is this sword? It’s like nothing I’ve ever seen before.” Because he’d certainly seen curved single edge blades. It would be, “why is this guy so terrible at using it?” Assuming he didn’t simply think, “filthy peasant” before running him through and going on with his day.