In my book that is unambiguously unethical and should get the contact fired. I am shocked to see this approach promoted in such a blasé manner.
If the point of the interview were "answer those questions AND know enough to answer the follow up questions" _once told what to expect and prep_, they’d be sharing those questions with all candidates. If you feel that saying to the interviewers "by the way, I did know this because [X] told me they’d be here" wouldn’t impact outcomes, then great. If you feel you’d need to hide that, then you’re aware this involves dishonesty - and if you still struggle to see how that’s unethical, lets just make sure we never need to work together.
Seeing as how you seem to prefer to let everyone else steal a march on you in interviews in the interest of "fairness", that's not likely to happen anytime soon.