This should be in the Stupid Questions thread, but: if the stuff she used sticks like that, and made the bathwater that blue, is your loo a deep shade of blue the whole time?
If you use it properly, no. You flush, squirt the stuff and let it sit for a while. Then you come back with a brush and scrub it a bit, then flush again. That should rinse almost all of it out. So this cleaner first used a product for cleaning things that it wasn't intended to clean, then failed to rinse it well, which is part of the "proper use" of the product.
Ah, got it. I'm just so used to toilet cleaners that don't really stick, and don't remove limescale that the thought of one that really works is quite exotic! I use the formic acid cleaner when I can't stand seeing the limescale any longer [usually takes a couple of years for it to build up, and is only on the bit around the bend that you can just
see/can't brush very easily], but that's a liquid that needs to be left overnight.
Thanks for the explanation.