This is actually a common issue with users that are new to Quick Base. Often, the problem is that the user is simply entering an email address or password they THINK is correct, but is often a letter/digit off. I would start by reviewing the email address that is in the system and checking with the user that they are using this exact email. If they have setup an actual Username, verify that with them as well. Using either of these should allow them to login with their correct password.
If they confirm the email/Username with you, they should be able to request a password reset directly from the Login page. If not, you can submit a support request and ask that their password be reset (if you have permission to manage your Quick Base account).
