Hah. This war has been started because of her. That's a well known fact to everyone. She's the big prize. Just why did she think....nevermind.
But also. Fuck if the victory counts afterwards or doesn't! Help them against the invader! They'll make up excuses , like 'If it wouldn't have been for the help of the witch...' anyways. Give the invaders a sound trashing! Keep the casualties of the defending soldiers low! There is NO reason not to! The nobles responsible won't learn their lesson anyways. Break their armies.
If the enemy is overwhelmed hugely, they will also capitulate faster and there will also be less casualties on the enemy side. It's the nicer thing to do.