After your client enters the email address (or User ID), the next dialog box that prompts for password has a link that says I forgot my Intuit password.  Clicking on it will trigger Intuit to send an email to the email address associated with her account.

The email she receives will have a link with a token for password reset.  On the webpage that the link opens, she should enter a new (instead of the old) password.  I've tested this multiple times over the years and it has always worked.

Now, if your client no longer has access to the email address associated with her account, that's where she may run into problems.