Yeah just tried it again now.
I deleted the partition again first, then when I got to the installer, it had created a new 50GB partition and mounted /var/crash and /var/log which can’t be unmounted (tried force unmount and all that jazz)
I recently tried this for the first time for my grandad on an old dying laptop of his which was struggling to run at any speed.
During the install it had already messed with the hard drive partitions in order to run the live environment, which is a big no-no for me.
The whole point of the live environment is it shouldn’t change the system until you try to install!
It also meant I no longer had a free partition to install to anymore so I couldn’t even get through the installer since I also couldn’t resize etc. because the partition was in use.
Been using Debian/Ubuntu based Linux for about 20 years and never seen this issue until Pop! OS
The cynic in me says maybe your coworker was just taking their time