Have them enter the Apple (iCloud) ID and click Forgot Password.
Apple ID - Manage
Apple ID – Forgot Password
If they are being told there isn't enough information to reset them, have them contact Apple ID Support.
If they don't know the answers, if the steps in the article below don’t work, have them contact Apple ID Support, although I’m not sure they will be able to help.
Apple ID - If you forgot the answers to your Apple ID security questions.
From the article below.
Without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account. These policies are audited and reviewed on a regular basis.
Security and your Apple ID.
Apple ID - Official Apple Support.
Contact Apple for help with Apple ID account security - Apple Support. (Link doesn't always load)
Get Support Work through the screens to get a chat option.
Apple Support schedule phone call
https://twitter.com/AppleSupport