Sabenaboy, without a reference, maybe this is a good starting point for discussion:
Phraseology "Squawk 1234" or "Squawk 1234 with ident" ask him what was said?
Point out to your co (gently) that maybe second guessing ATC is a bad habit to get into. On that basis - maybe he should learn the following is a good idea for the rest of his career - "Do as we say, not what you think we mean"