The thing about Crown Prince Randver secretly working with someone in Veanonia was just Prince Reigen's theory. It was probably the easiest theory his brain could come up with at the moment, and he couldn't even resist saying his theory in front of other people.
Prince Reigen didn't mean to say it because the matter related to his older brother was sensitive, and he didn't have enough evidence for that either. Moreover, he was with someone from outside of Breviel now, and such a thing would probably create some ripples between the two kingdoms.
Unfortunately, even if he didn't say his thoughts out loud, the person sitting next to him managed to hear his soft voice, and that person looked taken aback before immediately questioning him.