If we assume that ShadowHashData hash was introduced in High Sierra, I thought it was safe to assume that a crypt hash would only origin from a pre-High Sierra install. Unless, of course, they are used as some form of default value (such as when disabling the user)...

The factors that differed from my laptop, where it worked until I set a root password, as that her machine was a fresh install, and she is not admin (a managed machine).