She’s stupid. It would be different if she had thought they might be followed and left immediately.
But the enemy is already here.
They’re right on the doorstep and preparing their weapons/magic to attack.
Neither leaving immediately nor surrendering to the enemy would stop this attack.
So she’s essentially asking her new found friends and allies to be slaughtered defenselessly.
But ok, she’s a sheltered and/or clueless princess who doesn’t understand that.
MC should tho.
Yet he’s supporting her in this stupidity, which makes him even more stupid.