she does whatever's left of the damage that needs to be done to reach 0 hp
would you feel better if the damage counter wrote ∞ instead?
But remember that Rachel is indeed not even trying. She just sends her pets to fight for her, and when she's defeated, it's actually Nago that's defeated, not her, as he takes all the hits for her. I hope that someday we get an alternate Rachel character where it's actually her that fights and not her zoo.