MacBook won't boot after OS update - can access in Targed disk Mode

Hi,

A friend's 12 inch Macbook, early 2015 (MacBook (Retina, 12-inch, Early 2015) - Technical Specifications)

received an OS upgrade a few weeks ago, then never booted up to UI. Apple logo appeared, progress bar started a few times, but stuck.

I can access it in Target Disk Mode.

Not a pro Mac user here, tried what I googled so far:

• all kinds of recovery reinstall option combinations (Command (⌘)-R etc) How to reinstall macOS - Apple Support, original that came with the Mac, latest, etc
• Reset NVRAM, SMC

When I logged in single user mode, the date command said it is 21 Jan, 2021 (today it is 7 May, 2022). I set correct date with date -u, now correct date is retained between reboots.

When I open /var/logs in Targed Disk Mode, and order by modified, latest date is 23 March (the problematic update happened around this time), I cannot see recent boot logs.

Often it takes a lot of time for my Mac to see the friend's Mac in Targed Disk Mode, have to wait, or try several times.

I saved friend's disk in TDM to an external HDD with rsync, not sure the backup is perfect, but seems like we got the data.

Would like to revive the MacBook. Any advice on how to progress?