Faces in the Crowd: face blind and confused

In Julien Magnat’s Faces in the Crowd, a woman who barely survives an attack by a serial killer and wakes up in hospital with a head injury that leaves her “face-blind.” No longer able to recognize faces, she must navigate a world in which facial features change each time she loses sight of them. All the while the killer is closing in, determined to eliminate the potential witness. I am confused, if she can’t remember or facial features constantly change how the hell could she help at all? Sounds like she isn’t that much of a threat.

Source: STYD