It is only libel or slander if it is not the truth. Telling someone you think they are dishonest is legal. Telling someone they stole something from you and you did not could be slander if you can prove that it is the sole reason you were damaged from that statement. Very difficult to prove in the interview environment especially for pilots because so many aspects of an applicant are considered.

It is not legal to ask some particular questions but not illegal to share an opinion of someone. There are nasty people in this world and a few of them are pilots. Most airlines have given their pilots an opportunity to share their opinions of applicants to make the best decisions in choosing the best available people. None of that is illegal.