I'm pretty sure, from what you describe, your chick is out of immediate danger from this issue. I doubt it has an 'infection' of the kidneys (as in bacteria), as much as overburdened kidneys caused by processing the calcium in her layer diet.The damage is already done if the excess calcium got to their kidneys. They should not die from it, being that you've apparently gotten them switched to the correct feed now. It will take a bit, but they'll heal. They may be kidney-compromised, so make sure they always have water with apple cider in it, as I'd think they'll be a bit more sensitive to kidney issues.
Good luck, and good idea to talk with that friend when they get back!