MacBook Air can not log in to Apple Account

OK, a few things that were suggested in a similar thread:

• Boot up this Mac in Safe Mode. Try entering her Apple ID credentials.
• Boot up normally. This time, create a new macOS user account. Try the credentials again.
• If neither of those work, it may be possible that the wrong credentials are still stored in the Mac's Keychain. You can either use the Keychain Access app to try to locate and correct it OR simply use the app to delete the current Keychain.

Hope that helps. If not, someone else here may have additional suggestions. Or you can contact Apple Support for assistance.

Regards.

One thought is that perhaps the keyboard is acting up so that the keystrokes for the password aren't actually correct. If you have an external keyboard you can try with that MacBook, have her give that a go and see if that works or if the problem persists. And of course she should be certain she's not inadvertently typing extra spaces; that will cause a user ID or password to fail.

Hope this helps. Regards.