help with 3rd party sign in

Hi, I am trying to remotely help a senior with 3rd party sign in trouble. He is not teck savvy and due to Covid restrictions I cannot help him from his location.
He has been using a math/physics paid program called "Chegg" for some time and now it wants a verification code which he either does not know or does not know where to find it. His operating system is Ubuntu 16.04LTS, his browser is Firefox and his email is Gmail. Any help sorting this out would be appreciated.