So, if you were in my place what would you do?
You either keep to your no-raises-in-first-year policy, and explain that to future candidates so they won't even bother asking and her you tell now a stern "no" to the demand, or you speak to the higher ups and agree to drop the policy.
Keeping it obviously has upsides like more stability, but then you have to understand that loss of talent like this one will keep happening, because whenever you will find someone who trullytruly shines in their role, much higher than you expected, they will likely want to have their compensation fixed accordingly, and if they won't get it from you, they will get it somewhere else. And that is likely what has happened, she either already found a greener pasture, or is off to hunt for one.