Did the user sign up with a Google account, and forgot their Google password? If so, they would need to go the Google account recovery page:
https://www.google.com/accounts/ForgotPasswd
If they signed up using an e-mail address and password, they can just enter the e-mail address, and there should be a link that pops up offering them to reset their password.
If you PM me the name of the user in question, I can possibly give more specific advice or send out a password reset e-mail.
