MacBook Pro screen goes black after login

My dad's 15" retina early 2013 MacBook Pro (MBP) has the following problem. He's up to date with the latest macOS SIerra. It's happened twice now. He's using his Mac like normal. He is the only user defined and is thus an admin user. HIs login preferences are set to show his round icon and user name with the password field at startup. He set is password to blank, so he usually just types return to finish entering his password, and he is logged in. He shuts down his machine using the Apple > Shut Down menu. He can does this many times over the course of several weeks, and then the problem strikes.

When he starts his MBP, instead of user icon and password field, he's prompted for both a user name and a password. He types those in, and his desktop briefly flashes onto the screen, and then the screen goes black. The cursor is still visible and moves, but he can't see any GUI on the screen. He shuts down by pressing and holding the power key until power goes off. Pressing the power key never produces the customary shutdown-option dialog box. He doesn't have an external monitor to see if anything is appearing there.

The other odd thing the password he has to provide during the "broken" login is not blank. When he first installed macOS Sierra onto his Mac, he created his account and entered a non-blank password. After macOS was up and running, he went to the Users and Groups system preference and changed his password to be blank (just typed return).

My speculation is that something has corrupted his user settings causing this change in behavior. Booting into Safe Mode does not help--same login credentials needed (with actual password) and same black screen.

When this happened the first time, I was visiting over Christmas, and I was able to recover his data from a Time Machine backup. (The TM backup had actually backed up the corruption, and just migrating his data from the TM drive after a clean macOS install lead to the same black-screen behavior. I was able to extract just the data from the backup while booted from a third external HD that had macOS on it.)

Other facts:

1. When booted from a bootable external drive, the MBP runs fine.

2. When booted from the external drive and after installing a fresh copy of macOS onto the MBP's internal SSD, it boots up and runs just fine from the internal SSD.

3. When installing a fresh macOS, I created myself as the user. Later, I ran Migration Assistant to pull my dad's user account from the TM backup. Selecting his user ID at startup leads to the black screen. Selecting my user ID leads to normal operation. The problem is definitely tied to his account and doesn't seem related to the MBP's hardware itself.