He might not necessarily be complicit because he wants to be. The engagement is one thing, but he is helping get the bracelet off the hero, and he also might not be aware of all the evils going on in the kingdom. Being complicit or oblivious to evil deeds doesn't make someone not a good person. That and everyone in life has failings, although I'm a bit cautious to lump obliviousness in as a failing when it's impossible to fully control you're own intake of information.
All of that being said (also sorry if that came off as grand-standy), the author could be setting duke guy up as a betrayer or double-agent.
Think it's more akin to ignoring a person's cries for help as they are being mugged/raped in an alley. Most want to help, but don't want to be dragged into the situation.
And while the above example is a bit extreme outside of the big cities, most of us should be more familiar with rampent bullying and harassment that happens in schools and work.
While we do have systems that are ment to protect us from repercussions of reporting such crimes. The more likely result is being dragged into the situation and also being targeted.
The enshitification of the world started long before the interwebs, and it really does touch on all levels of our lives. You either keep your head down, or try your luck and find enough like minded people to with similar values so as your not just another nail that someone with a hammer can beat back into place.
So wrapping back around to the noble dud. He may have enough power to truely stand upto the king, but if there are too many other nobles that support the king, the best he can do is subverting his rule some other way. Like having his little sister marry the crown prince, while the hero marries the princess.