I'd get her to the vet they may have to sedate her but you want it out before it goes into her system and causes metal poisoning.
I doubt you could use a magnet I don't know what else can be done.
https://www.backyardchickens.com/threads/another-bumblefoot-post.1492463/#post-24896429
Pick up activated charcoal it’s a good one to have on hand. For toxins
Is there swelling or redness any where from her foot up to top of her leg? Can you get a picture?