How do I delete files on a bootable osx Catalina disk from another bootup disk? My friend's 2016 iMac keeps crashing on restart after trying to install the latest Catalina update. The problem seems to be not enough disk space, he only has about 16 GB free on his internal Mac HD. 

I had him boot up in recovery mode and install a fresh system on an external HD. Now I am trying to get into the MacHD drive to clean up some space but I can't get in to his files. Under 'Users' I just see 'Shared'. Nothing shows up in his application folder either. I am able to see some system files but not the personalised ones. 

I tried changing permissions for the Mac HD to read &amp; write but still no lock. I had him do it first over the phone but he is a total novice so ending up using Anydesk to access his computer remotely. Any suggestions of how I can clean up some space? Or can I cancel and ongoing update on restart with a key command?
Is your Mac running a beta version of macOS? There is a known bug where the system crashes with lower amounts of space in Catalina. You should try resetting the NVRAM first. Then, you can also try a reset of the SMC for power related issues. To delete files off of the Internal Drive, boot up on the bootable drive and go to Finder and click on the Internal Drive. If FileVault was enabled, then macOS will prompt for the login password of one of the users on the problem instance of macOS. This will unlock and mount the drive. Then, you can use Finder and Terminal to navigate the files and delete.