Ok, so this was a decades-old foot corn. It was covered by a thick layer of hard skin.
(Longer video of said callus removal here)
It was very painful as it was smack bang in the middle of her upper foot.
This was caused by a pronounced bunion which meant most of the pressure from walking was put right through this central point.
With these types of foot corn, they become more like a giant disc of hard skin as opposed to a corn.
In other words, removing them needs lots of chipping away as opposed to "popping out".
Sometimes, foot corns this old need a re-treatment in a few weeks to get on top of it (as well as offloading with an insole going forward).
(Plus a good healthy dollop of Gehwol Callus Cream to stop it hardening up again)