This problem has been resolved, but it's occurred to me to post a heads-up, because it was a never before heard of situation.
My daughter-in-law's MacBook Air booted to a prohibited sign after being upgraded to High Sierra, and since I needed to use it while my MBP was in the shop, I got to troubleshoot it.
My first move was an option boot to see how the machine was configured, and I found an anomalous installer volume and a user account.
I selected the user account, and when it booted into High Sierra with no further ado I rebooted to see what would happen if I selected the installer, and it booted to...the prohibited sign.
Thinking the matter through, I guessed that what had probably happened was the upgrade had completed successfully, but the installer had never actually quit, hence the prohibited sign, i.e. since it hadn't "withdrawn," it was the default boot volume, but I couldn't boot into it, because it had finished its job.
My solution was to rerun the installer, and after it had completed its job, the MBA booted, as expected, into the user account.
...good thing, too, because my other option was an MBP that was maxed out at El Cap and was incapable of runnig either my High Sierra or Mojave clone.
The new Great Equalizer is the SEND button.
In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire