Well this guy could not be more obvious he is a bad guy could he... Honestly she should just order for his capture and imprisonment under the suspicion of treason. She is still nobility after all. But I will let this one slide since she is basically disliked by most higher nobility, so guards may not follow her command.
However, this guy just walks into the battle to just stand around and talk while immediately pointing blame towards someone besides himself. Even a 3 year old would know that now is not the time for that. Only someone guilty would try to push blame right off the bat before they look to help.
His argument is flawed logically too. He would rather believe that something is impossible after seeing potentially the exact outcome than accept that just because it had yet to happen, does not mean it cant happen. There is even a book about said thing happening.
Normally his strange behavior in a time of crisis could be overlooked by being emotionally overwhelmed, except he cant be considered that. Even if he didnt speak about their expulsion, simply his calm demenor towards the situation is suspicious. Almost as if he knew this would happen at this moment. He only is showing hostility towards the princess.
But in the end, what does it matter. Either this guy gets arrested, killed, banished, or everything he wants. Princess either gets people to come to her side and is accepted by other nobles, or just fucks off with MC and his harem. Or even both happen.