Doesn't matters what he says the only thing that awaits Kazuya is a fat L from the NTR that will come his way
Say yes:
The king becomes his enemy, so now he has to either try to usurp the throne or live in exile by becoming a societal pariah as the guy that ruined the realm by stealing the princess away from a political marriage, possibly earning the ire of the neighboring kingdom too.
But at least he does it for love, and gets to live happily with his childhood friend right?
Wrong!!! while that shit is going on, the protagonist is banging his wife, because he is getting NTRd now that she is married and a valid target for the wife charmer skill, she joins the new hero party, and he is left maidenless and with 2 kingdoms hunting his ass, everyday he has to live on the run, while his wife says that he loves him but that the hero's dick is better so she just has to go to take down the demon lord
Says no:
He gets to keep his current life, maybe meet someone else, his childhood friend marries some random prince, and then it is the random prince the one getting NTRd by the protagonist who is the one that will ultimatelly be fucking her anyway, since he technically turned her down, it is some other dude the one getting the business this time
Both options sucks, but saying no sucks less, the only way for saying yes working out, would be for him to, take the throne by beating the king, fend off the neighboring kingdom's prince, and also beat the hero and his wife charmer ability by avoiding getting NTRd, all at the same time, while having the childhood friend de-buff of them never winning
I don't see it happening, he has to decline, this is a hard pass, no childhood friend in manga can win with those odds, specially with a walking NTR magical bomb as the main character next to him