...clay is not harmful to kidneys, but kidneys should be actually functional when using ANY type of oral/internal therapy.
Kidneys that don't filter... are kidneys that do not filter. Kidneys handle PARTICULATES differently than dissolved solids. So, to say that the kidneys are still able to filter some dissolved solids, but not particulates, is not a reflection on clay itself, but on the kidneys' ability to deal with colloidal substances (it wouldn't just be clay).
Furthermore, very, very little, if any, smectites would ever need filtration from the kidneys as the particles don't make it past the liver.
In fact, this particular nurse may have been noticing something like silicon dioxide, or another non-smectite mineral. It's very hard to say.
Interestingly, properly made micronized zeolite... with particles that ARE small enough to enter the bloodstream... may have a positive long-term impact on restoring function to ailing kidneys....but it can take a long time, months to years.
In traditional medicine, external clay (clay poultices) is part of the solution, using clay packs over the kidneys to stimulate function.