when you defeat her you get a chance to get a card. If you obtain a card, then it will pop up on the screen showing the card you earned. Like when you get any other non treasure card card.
It doesn't happen very often. She drops all schools equally, so most spells she drops will be off school and not useful to you. It seems that drop rates for the most prized spells are a bit lower than the others. By all accounts you have to defeat her literally hundreds of times to get all the spells for your school. Crafting her spells is a pain but way easier than getting them to drop.
