I've looked it up and the doc suggested resetting the SMC which I've done but no change. She has to shut down and log in again and password accepted.

Has anyone come across this before?

Bluetooth or wired keyboard?

Can the password be typed but the Mac doesn't recognise it or is it an inability to enter anything into the password field?

No external keyboard. She's using the one on the laptop

Sounds like a hardware issue with the keyboard, maybe a particular key is sticky or has a connection problem? Could she try changing the login password to another one that doesn’t use any of the same keys as in the existing password? Just to test if it’s one of those keys.

Or, if she doesn’t want to change the password, try connecting a USB keyboard to see if that behaves the same, if it does then likely a software issue

I'll suggest that. I did try and book some time in with the laptop today but she's now busy until Monday. I may be back!!