If the login credentials are correct and the client cannot login, this most likely has to do with either the manner in which the client is inserting the credentials or settings within their computer's browser.
There are two situations.
A) If the client uses the copy/paste commands to insert the username and password into your login screen, they may be unable to login for reasons explained below.
B) If they manually type in the username and password, it should work perfectly.
This has to do with either (1) a setting in their browser or (2) when they highlight/copy, often the computer highlights "hidden" characters and empty spaces (naked to their eyes) and they arbitrarily get pasted in, thus causing the username/password to be incorrect.
Be sure that your client is typing in their credentials and be sure the client knows they are both case and space sensitive.
It should work perfectly.
If the above does not make things right for your client, you may ask them to clear their cookies/cache in their browser. They can do this by opening their browser, clicking on TOOLS > OPTIONS and checking the option for cache and cookies to clear private data. Depending on the browser, tell your clients to follow the instructions presented in this link:http://www.aboutcookies.org/default.aspx?page=2