Have a look into this guide for bumblefoot treatment: https://www.backyardchickens.com/articles/treating-bumblefoot-a-simple-surgical-approach.75146/
Quite ften the surgical approach is successful, but sometimes you would need a round of antibiotics as well.
In your picture I can see that she must have stepped on something like a thorn or similar that got stuck right in the middle of her foot pad. If you look very close, you might even see the tip of it still sticking out a bit.